Ask 10 people what makes steel “stainless” and most will say that it doesn’t rust. This is true to a certain extent, but there are more than 150 stainless steel grades and selecting the wrong grade can be one of the costliest errors that a project can encounter. We offer stainless steel pipes, tubes, fittings and fasteners in the entire spectrum of commercial grades and the question that is most often asked by engineers or buyers is which grade should be used? This guide eases up the families, the individual grades within each family, and a match for a grade in an actual application instead of the guesswork.

What is Stainless Steel?

Stainless steel is just steel that has a minimum of approximately 10.5% Chromium within the alloy. It is not the properties of the steel that stop it from corroding, it is a thin coating of chromium oxide which forms on the surface of the steel when it reacts with oxygen. As long as there is enough oxygen and chromium around, the surface would be damaged and it would be regenerated automatically; therefore, stainless steel is said to be self-healing. Add more chromium, some nickel, molybdenum, or nitrogen and you have two very different grades, each with a different level of corrosion resistance, strength and cost. I think that’s the only point that exists for the existence of grades with no single formula that works equally well in a commercial kitchen, chemical tank, and surgical instrument.

Different Families of Stainless Steel

Stainless steels are classified into five groups based on the crystal structure that the alloy can adopt. Learning the family can offer you a lot of insight into a grade prior to looking at the actual number.

Austenitic Stainless Steel

Some of the commonly available types of stainless steel are the austenitic variety, which constitute up to 70% of stainless steel products and 304 & 316 are some of the well-known examples. Austenitic type has a face centred cubic crystalline structure which is stabilized by the presence of nickel and this crystalline structure provides good formability, weldability and non-magnetic nature of these types of stainless steel. Although the austenitic type does not harden by heating process, 16 to 30% chromium present in these different ranges of stainless steel products provide the maximum corrosion resistant properties among all the types of stainless steel families.

Ferritic Stainless Steel

Ferritic grades use nickel to increase chromium, and are generally between 11% and 27% chromium with very little nickel which reduces the cost but also lowers corrosion resistance below that of the austenitic grades. Ferritic steels can be easily identified from the austenitic grades on the shop floor due to their body centered cubic structure which makes them magnetic. These are the most common grades (430 and 409), and are often used in automotive trim and exhaust systems, where the cost is more important than marine-grade corrosion resistance.

Martensitic Stainless Steel

Martensitic grades are the only family where the heat treatment can be used to harden the steel, and hence they are used to make cutlery, surgical instruments and turbine blades. They are strong and wear resistant due to their higher carbon content, but have less corrosion resistance and weldability than austenitic grades. This family is controlled by grades 410 and 420, both of which are magnetic and are easily noticeable by the need for preheating and post-weld heat treatment when welding without it.

Duplex Stainless Steel

Overall, duplex grades are roughly 50/50 austenite and ferrite, and possess properties of both phases, including approximately double the strength of standard austenitic grades and good resistance to stress corrosion cracking. Grade 2205 is a typical reference in this family and super duplex grades (such as 2507) further extend corrosion resistance for offshore and chemical processing service. Whether you need to relax about using custom mill lead times, we have both our Duplex Steel S31803/S32205 and Super Duplex S32750/S32760 pipe and tube lines in stock and available to use in your projects as soon as they are needed.

Precipitation-Hardening (PH) Stainless Steel

They are alloyed with small amounts of copper, aluminum or titanium and age hardened to allow those elements to precipitate as hard intermetallic compounds which impede the movement of dislocations in the crystal structure, resulting in a strength significantly higher than standard austenitic grades. Grade 17-4 PH is the most widely used, and is used in aircraft and precision tooling applications where both strength and corrosion resistance are important. PH grades are not seen often in the general industrial piping, but do play a part in any application where both high mechanical loads and corrosive exposure are present.

Different Grades of Stainless Steel 

The stainless steel products are manufactured in a variety of forms and grades according to the industries. Some of the most common grades of stainless steel are listed below. 

Grade 304 / 304L

Grade 304 is the default stainless steel for a reason: 18% chromium and 8% nickel give it strong all-around corrosion resistance, excellent formability, and reliable weldability at a moderate cost. The low-carbon 304L variant resists sensitization during welding, making it the safer choice for heavily welded assemblies. We supply 304 across tube fittings and the broader stainless product line for buyers who need it in bulk with full material traceability.

Grade 316 / 316L

Grade 316 adds 2 to 3% molybdenum on top of the 304 composition, and that addition is what gives it meaningfully better resistance to chlorides, saltwater, and industrial chemicals. It’s the standard choice for marine hardware, pharmaceutical processing, and any chemical environment where 304 would eventually pit. Our 316 tube fittings range covers both 316 and the low-carbon 316L variant for welded systems.

Grade 310 / 310S

Grade 310 pushes chromium up to roughly 25% along with 20% nickel, which delivers outstanding resistance to oxidation and scaling at high temperatures, well beyond what 304 or 316 can handle. It’s the grade of choice for furnace parts, kilns, and heat treatment equipment. We supply 310 through our stainless steel valve and fitting range for high-temperature service.

Grade 321 and 347

Both grades are stabilized against intergranular corrosion after welding, 321 with titanium and 347 with niobium, which makes them the preferred choice for expansion joints, exhaust systems, and chemical process equipment that sees repeated heat cycling. Neither is a general-purpose grade; both exist specifically to solve the weld-decay problem that standard austenitic grades face at elevated temperature.

Grade 410 and 420

These martensitic grades trade corrosion resistance for hardness. Grade 410 heat-treats well for shafts, valves, and fasteners exposed to mild corrosive conditions, while 420’s higher carbon content makes it the standard for cutlery, surgical instruments, and cutting tools where edge retention matters more than chemical resistance.

Grade 430

A ferritic grade with no nickel, 430 is the economical choice for kitchen appliances, automotive trim, and dishwasher linings where moderate corrosion resistance and a bright finish matter more than the extreme durability of an austenitic grade.

Duplex 2205 and Super Duplex 2507

These grades combine strength and corrosion resistance in a way no single-phase stainless steel can match, which is why they’ve become standard in offshore platforms, desalination plants, and pressure vessels handling aggressive chemicals. The tradeoff is cost and more demanding welding procedures, but for the right application, duplex grades reduce wall thickness and overall weight compared to an equivalent austenitic design. Our duplex and super duplex lines are stocked specifically for these demanding sectors.

Stainless Steel Applications by Industry

Food and beverage processing. Grade 304 dominates here because it needs no coating or plating, resists the frequent washdowns and cleaning agents used in food production, and can be polished to a hygienic finish for equipment like hoppers, conveyors, and tanks.

Medical and pharmaceutical. The same cleanliness properties that make 304 useful in food service make austenitic grades, particularly 316L, the standard for surgical instruments, medical devices, and pharmaceutical process equipment where sterility and chemical resistance both matter.

Oil, gas, and marine. This is duplex and super duplex territory, alongside 316 for less extreme service. Chloride exposure from seawater and produced fluids demands the pitting resistance these grades provide, which is why our duplex and super duplex products see heavy demand from offshore and petrochemical buyers.

Construction and architecture. Stainless steel’s low maintenance and corrosion resistance justify the higher upfront cost on landmark buildings and structural applications, with 304 and 316 both common depending on whether the structure faces coastal or industrial exposure.

Automotive. Ferritic grades like 409 and 430 dominate here, chosen for their lower cost and adequate corrosion resistance in exhaust systems, trim, and catalytic converter housings where extreme corrosion resistance isn’t required.

How to Choose the Right Stainless Steel Grade

Grade selection comes down to weighing a handful of factors against each other, since the grade that wins on one measure often loses on another.

• Corrosion environment. Match the grade to the specific corrosive agent, not just “how corrosive” the environment is in general. Chlorides call for molybdenum-bearing grades like 316; high heat calls for 309, 321, or 347; general atmospheric exposure is often fine with 304.
• Mechanical requirements. If the part needs to be hardened, only martensitic or PH grades qualify. If it needs maximum strength without heat treatment, duplex is usually the answer.
• Fabrication method. Heavy welding favors austenitic grades; parts requiring hardening favor martensitic; cost-sensitive stamped parts often work fine in ferritic grades.
• Budget. Duplex and high-nickel austenitic grades cost meaningfully more than standard 304, so it’s worth confirming the application actually needs that extra performance before specifying it. Our guide on common mistakes when selecting stainless steel flanges covers several real cases where over-specifying or under-specifying grade caused costly rework.

Why Source Stainless Steel from Aashish Metals

Aashish Metals stocks and supplies stainless steel across the grades covered in this guide, including 304, 304L, 310/310S, 316, 316L, 316Ti, 317L, 321, 347H, and 904L, alongside duplex and super duplex grades, in pipes, tubes, fittings, valves, and shims. Every product ships with full material test certificates and traceability, and we export across more than 50 countries with stock ready for fast dispatch rather than waiting on a fresh mill run. If your project also needs matching flanges in the same grade, our flange range is built to the same material and pressure-class standards. You can learn more about Aashish Metals or contact our team directly for grade-specific technical guidance on your next order.

Conclusion:

Stainless steel grade selection isn’t about finding the “best” grade in some abstract sense, it’s about matching a specific set of properties to a specific application, environment, and budget. Understanding the five families and how the common grades within them differ makes that decision far easier, whether you’re specifying pipe for a chemical plant or fittings for a food processing line. Aashish Metals supplies the full range of grades covered in this guide, backed by material documentation and decades of export experience. Reach out to our team for help matching the right stainless steel grade to your next project.

Frequently Asked Questions

Selection of the right stainless steel grade begins with knowing its chemical composition. Different stainless steel grades have varying amounts of chromium, nickel, molybdenum, carbon, and other alloying elements that contribute to the material’s corrosive resistance, strength, and suitability to be welded in different applications. Even small variations in chemical composition could make one material fit for particular use while others do not. The Stainless Steel Composition Chart provides the standard chemical composition of the more popular stainless steel types such as 201, 304, 304L, 316, 316L, 310, 321, and 904L. This will facilitate comparison of materials and checking mill test certificates (MTCs). Kalpataru Piping is a supplier of high quality stainless steel pipe, tubes, fittings and flanges available in these grades of stainless steels.

Reading the Composition Chart Before You Order

Check the chemical composition of the selected stainless steel grade against the applicable specification, such as ASTM A240, before making the selection to determine if the material is suitable for the project. The range of values is used to express the composition of a material. The material is deemed to be compliant if it is within the range. Also note the UNS designation which is unique to each alloy, while the names 304, 18-8, and 316 are common terms used in the industry. To read the chart, first compare chromium and nickel levels -chromium and nickel are the factors that determine corrosion resistance and ductility. Check for molybdenum, which is included in the 316 series and the 904L, which offers greater chloride resistance. Lastly, check carbon for weldability and titanium for stabilized Grade 321 for high-temperature applications.

Stainless Steel Composition Chart: 201, 304, 304L, 316, 316L, 310, 321 & 904L

Grade by Grade: What Each Composition Means in Practice

201 Stainless Steel

Grade 201 (UNS S20100) is a cost-effective alternative to 304 stainless steel, developed by reducing nickel and increasing manganese and nitrogen. It contains 16.0–18.0% chromium, 3.5–5.5% nickel, and 5.5–7.5% manganese, providing good strength, formability, and moderate corrosion resistance. However, its lower nickel content makes it less resistant to chlorides and acidic environments than 304 or 316. Grade 201 is commonly used in kitchen equipment, decorative trim, appliances, and light structural stainless steel applications, where high corrosion resistance is not the primary requirement.

304 Stainless Steel

Grade 304 (UNS S30400) is the most widely used austenitic stainless steel due to its excellent balance of corrosion resistance, strength, and weldability. It contains 18.0–20.0% chromium and 8.0–10.5% nickel, making it suitable for a wide range of industrial and commercial applications. Carbon is limited to 0.08%, although prolonged welding can increase the risk of sensitization. Grade 304 is widely used for pipes, tubes, tanks, flanges & fittings, food processing equipment, and architectural structures exposed to mild or moderately corrosive environments.

304L Stainless Steel

Grade 304L (UNS S30403) is the low-carbon version of 304 stainless steel, with a maximum carbon content of 0.03%. This lower carbon level minimizes carbide precipitation during welding, reducing the risk of intergranular corrosion. Its chromium and nickel content remains nearly identical to standard 304, so corrosion resistance is maintained. Although its strength is slightly lower, 304L is the preferred choice for pressure vessels, storage tanks, welded piping systems, and fabricated equipment where extensive welding and long-term corrosion resistance are essential.

316 Stainless Steel

Grade 316 (UNS S31600) offers improved corrosion resistance over 304 by adding 2.0–3.0% molybdenum and increasing nickel to 10.0–14.0%. Chromium ranges from 16.0–18.0%, providing excellent resistance to pitting and crevice corrosion in chloride-rich environments. This makes 316 suitable for marine, offshore, pharmaceutical, food processing, and chemical industries. Its superior resistance to seawater, process chemicals, and de-icing salts makes it the preferred grade whenever standard 304 may not provide sufficient corrosion protection.

316L Stainless Steel

Grade 316L (UNS S31603) combines the corrosion resistance of 316 with a reduced carbon content of 0.03% maximum, improving weldability and preventing sensitization in welded areas. Chromium, nickel, and molybdenum levels remain almost the same as standard 316, ensuring excellent resistance to chlorides and aggressive chemicals. Because of its reliable performance after welding, 316L is commonly specified for marine structures, offshore equipment, pharmaceutical plants, 316L compared against 904L is typically specified by default rather than treated as an optional upgrade over standard 316 chemistry.

310 Stainless Steel

Grade 310 (UNS S31000) is designed for high-temperature service rather than chloride resistance. It contains 24.0–26.0% chromium and 19.0–22.0% nickel, giving outstanding resistance to oxidation and scaling at temperatures exceeding 1000°C. Higher silicon content further improves heat resistance, while carbon is maintained at about 0.25% to enhance creep strength. Grade 310 is widely used in industrial furnaces, heat treatment equipment, kilns, combustion chambers, and high-temperature processing systems, where conventional grades such as 304 or 316 cannot withstand prolonged heat exposure.

321 Stainless Steel

Grade 321 (UNS S32100) is a titanium-stabilized version of 304 stainless steel developed for high-temperature welded applications. It contains approximately 17.0–19.0% chromium, 9.0–12.0% nickel, and titanium, which prevents chromium carbide formation during welding. Unlike 304L, stabilization is achieved without significantly reducing carbon content. Grade 321 provides excellent resistance to intergranular corrosion after welding and performs well at elevated temperatures. It is commonly used in exhaust systems, aerospace components, heat exchangers, and process equipment operating between 425°C and 900°C.

904L Stainless Steel

Grade 904L (UNS N08904) is a high-alloy austenitic stainless steel developed for extremely corrosive environments. It contains 19.0–23.0% chromium, 23.0–28.0% nickel, 4.0–5.0% molybdenum, and 1.0–2.0% copper, providing outstanding resistance to chlorides, sulfuric acid, and phosphoric acid. Carbon is limited to 0.02%, further improving corrosion resistance after welding. Grade 904L is widely used in chemical processing plants, flue gas desulfurization systems, fertilizer production, seawater equipment, and offshore applications, where even 316L may not provide adequate long-term performance.

Common Mistakes to Avoid When Comparing Stainless Steel Grades

• Don’t assume 316 is always better than 304
  Grade 316 offers better corrosion resistance only in chloride and marine environments. For general-purpose applications, 304 is often the more economical and suitable choice.
• Don’t treat the “L” grades as weaker materials
  The “L” suffix (304L, 316L) indicates low carbon content, which improves weldability and reduces the risk of intergranular corrosion after welding.
• Don’t ignore the alloying elements
  Elements listed under “Others” are important. For example, titanium gives 321 excellent high-temperature weld stability, while copper improves 904L resistance to strong acids.
• Don’t choose only based on corrosion resistance
  If your application requires higher strength along with corrosion resistance, a duplex stainless steel grade may be a better choice see our duplex stainless steel composition guide.

Final Thoughts

A composition chart is only useful if it changes what you actually order, so treat every percentage in this table as a decision point, not a reference to skim past. Chromium and nickel set the corrosion and ductility baseline, molybdenum decides whether chlorides will be a problem, carbon decides whether heavy welding will be safe, and elements like titanium or copper mark a grade built for a specific service condition. Whether the application calls for the economy of 201, the reliability of 304, the chloride resistance of 316 or 316L, the heat tolerance of 310, the weld stability of 321, or the acid resistance of 904L, matching chemistry to service environment against a proper dimension and composition reference is what keeps stainless steel living up to its name for the full design life of the equipment.

Frequently Asked Questions

An order of a 6-inch pipe without detailing its complete dimensions is a costly mistake. Even though the nominal dimension will be similar, OD, WT, ID, and Length details will help one to know if the pipe can serve its purpose. These dimensions of the pipes are indicated by standards such as NPS, DN, and Pipe Size Chart to help make sure uniformity throughout the manufacturing industry. The knowledge of these standards helps in understanding the pipe sizing chart and selecting the right pipe size and schedule. Whether you are sourcing pipes for your engineering and projects or for industrial or structural applications, it is necessary to have proper dimensions specified to get success in your project. Kalpataru Piping manufactures and distributes industrial piping in standard sizes and schedules to suit your engineering projects around the globe.

The Building Blocks of a Pipe Dimension

The four essential pieces of information which make up a pipe specification are: Outer diameter (OD), Wall thickness (WT), Inner diameter (ID), and Length. The OD is not affected by the schedule and is the same for pipes of a given nominal size, even when they come in different wall thicknesses, which means that the same standard flanges and fittings can be used on pipes of different wall thicknesses. The thickness of the wall varies according to the schedule and directly influences the pipe’s pressure rating, strength, weight and cost. The inner dia (also called bore) is the OD – 2 x wall thickness. It is not specified on the purchase order, but it is crucial as it dictates the pipe’s flow capacity. The length of pipe is usually provided in SRL (Single Random Length) or DRL (Double Random length) as per application and manufacturing standard.

Once you have OD, WT, and length locked down, everything else, including the theoretical weight per unit length and the maximum working pressure, can be calculated. That is precisely why pipe weight and pressure formulas always start from these three inputs, a point worth remembering the next time you need to cross check a steel weight calculation against a mill test certificate.

NPS and DN: Two Naming Systems for the Same Pipe

Nominal Pipe Size (NPS) and Nominal Diameter (DN) are common pipe size designators but neither is the actual outside diameter (OD). NPS (North American system) is measured in inches (e.g., NPS 2, NPS 8). In NPS 12 & below, the NPS value will be different from the actual OD. For example, NPS 6 has an actual OD of 6.625 inches (168.3 mm). The NPS value is equal to the pipe’s outside diameter (in inches) if NPS is 14 or higher.

The metric version of NPS is DN, which is measured in millimeters (e.g., DN 50 or DN 200). It is a nominal size as with NPS and not an actual OD size. Because both NPS and DN are used extensively on engineering drawings, purchase orders and specifications all over the world, it is helpful to be aware of the correlation between these, so that the appropriate pipe size can be selected and mistakes in ordering that size are avoided.

For DN 100 and above, the conversion becomes far simpler and follows two approximate rules:

• DN is roughly equal to NPS multiplied by 25
• NPS is roughly equal to DN divided by 25
  

These are approximations built around the exact relationship that 1 inch equals 25.4 mm, and they hold well enough for identification purposes, though the true governing figures always come from the OD tables in the relevant ASME standard rather than from the rough conversion.

Pipe Schedule: What the Number After “Sch” Actually Means

Pipe Size Chart and  the wall thickness of a pipe, while NPS and DN identify its nominal size. It is written after the pipe size, such as NPS 4 Sch 40 or Sch 80. The schedule is based on the pipe’s design pressure and the allowable stress of the material, making it a standard way to indicate pressure-handling capability rather than just thickness. Common schedules include Sch 5, 10, 20, 40, 80, 160, and stainless steel schedules like 5S, 10S, 40S, and 80S. Higher schedule numbers mean thicker walls, smaller inside diameter, greater weight, and higher pressure capacity. If your team regularly works with the lower and middle end of this range, our breakdown of what a Schedule 40 steel pipe actually represents in practice is worth a closer look.

Pipe Size Chart in MM and Inches: OD, ID & Wall Thickness

With the naming systems and the schedule concept out of the way, here is how the numbers actually line up for some of the most frequently ordered nominal sizes, shown in both millimeters and inches, across Schedule 40 and Schedule 80, the two schedules most commonly specified for general industrial and process piping.

When reading this table, remember that the outside diameter (OD) stays the same for the same nominal pipe size, regardless of the schedule. Only the wall thickness and inside diameter (ID) change. A thicker wall results in a smaller ID, which affects the pipe’s flow capacity. For larger pipe sizes (above NPS 12), some schedules may have the same wall thickness.

This chart covers the two most requested schedules for general use, but the complete dimension standard runs from Sch 5 through Sch 160 for carbon and alloy steel pipe, and from Sch 5S through Sch 80S for stainless steel. For a size range built specifically around stainless grades, our dedicated stainless steel pipe dimensions chart lays out the 5S through 80S series in full, and our carbon steel pipe guide covers the equivalent detail for carbon grades.

The Governing Standards: ASME B36.10M and ASME B36.19M

The dimensions in the chart are based on two ASME standards. ASME B36.10M covers carbon steel and alloy steel pipes, while ASME B36.19M covers stainless steel pipes. Stainless steel uses schedules such as 5S, 10S, 40S, and 80S, and some wall thicknesses differ from carbon steel pipes of the same schedule. However, the outside diameter (OD) remains the same for the same nominal pipe size (NPS), making them compatible for standard fittings in most applications. It is only the wall thickness tables that diverge in places, which is worth keeping in mind if a project calls for switching material grade mid procurement on an existing piping system, whether it is carbon steel pipe or stainless steel pipe and tube.

How Wall Thickness Drives Pipe Weight

Wall thickness does more than set the pressure rating. It is the single biggest variable in how much a run of pipe will weigh, and weight feeds directly into freight cost, handling requirements, and, for many buyers, the per meter or per foot price quoted by the mill.

The theoretical weight of a straight length of pipe is calculated from its cross sectional area of steel (derived from OD and WT) multiplied by the material density and the length. Two pipes of identical OD and identical length, one at Sch 40 and one at Sch 80, will differ in weight by a wide margin because the heavier schedule is carrying substantially more steel in its wall, not because it is somehow a different size. This is why quotations that only state a nominal size without a schedule are effectively incomplete, since price and weight cannot be pinned down from NPS or DN alone.

For buyers who need to check delivered weight against theoretical weight, or who are estimating freight before a purchase order is finalized, running the OD and WT figures from a chart like the one above through a proper pipe weight calculator is far more reliable than eyeballing it, particularly for larger diameters where small errors in wall thickness compound quickly across a long run.

How to Correctly Specify Pipe Dimensions on a Purchase Order

A purchase order that only says “6 inch pipe” leaves too much open to interpretation. Industry practice has settled on a handful of accepted formats, and using one of them removes ambiguity entirely.

By outer diameter and wall thickness: written as OD x WT, for example 168.3 mm x 7.11 mm, or in inches, 6.625 in x 0.280 in. This format is unambiguous because both values are absolute measurements rather than designators, and it is the preferred format for custom or non standard wall thicknesses that fall outside the schedule tables.

By nominal size and schedule: written as NPS x Schedule, for example NPS 6 Sch 40 or DN 150 Sch 40. This is the most common shorthand in industrial procurement because it references a published standard table rather than requiring the buyer to state every dimension explicitly.

By nominal size and weight class: written as NPS x weight class, for example NPS 6 STD or NPS 6 XS. This older convention predates the schedule numbering system and is still used interchangeably with Sch 40 (which corresponds to STD for most small and mid range sizes) and Sch 80 (which corresponds to XS for the same range), though the two systems diverge at some larger diameters, so it pays to confirm which one your supplier is quoting against.

By outer diameter and weight per unit length: common in the Americas, written as OD in inches followed by a weight in pounds per foot, for example a 3.5 inch OD pipe quoted as 16.8 lb/ft. This format is really a proxy for wall thickness, since weight per foot is derived directly from OD and WT, and it is worth converting back to an actual wall thickness figure before finalizing an order if your project specification calls for a specific schedule.

Whichever format you use, always pair it with the governing standard (ASME B36.10M or B36.19M), the pipe end type (plain end, beveled, threaded), and whether the pipe is seamless or welded, since dimensional tolerances and, in some cases, minimum wall thickness rules differ between the two. If your spec allows either construction, it is worth reading through the practical difference between ERW and seamless pipe before locking in the order, since the choice affects both tolerance and, in certain services, allowable pressure derating.

Common Mistakes Buyers Make When Reading a Pipe Size Chart

• Treating NPS as a literal diameter. As covered earlier, NPS 8 pipe is not 8 inches across; it is 8.625 inches. Assuming otherwise leads to fit up surprises with flanges and fittings ordered separately.
• Ordering by nominal size alone, with no schedule specified. Without a schedule or an explicit wall thickness, a mill cannot confirm weight, pressure rating, or even a firm price, and the order will bounce back for clarification, costing time.
• Assuming Sch 40 and STD are always identical. They match for most common sizes but not universally, particularly above NPS 10, where the two systems can specify different wall thicknesses for the same nominal size.
• Mixing carbon steel and stainless steel schedule tables. Because B36.19M’s “S” schedules diverge from B36.10M at several sizes, pulling a wall thickness figure from a carbon steel chart and applying it to a stainless steel order (or vice versa) can produce the wrong pipe entirely.
• Overlooking tolerance. Every OD and WT figure in a size chart is a nominal value; actual manufacturing tolerances (which vary by whether the pipe is seamless or welded, and by the governing product standard such as ASTM A106, A53, or A312) permit small deviations, and tight fit up applications should always account for this rather than assuming the chart figure is exact to the decimal.

Final Thoughts

A pipe size chart looks like a simple reference table until you actually have to place an order against one, at which point every column, OD, DN, NPS, schedule, and wall thickness, turns out to carry real consequences for fit, pressure rating, weight, and cost. Reading it correctly comes down to remembering three things: OD is fixed for a given nominal size, wall thickness is what actually changes with schedule, and NPS or DN are designators rather than literal
measurements for most small and mid range sizes. Whether you are specifying carbon steel, alloy steel, or stainless steel pipe, stating dimensions in one of the recognized formats, OD x WT, NPS x Schedule, or NPS x weight class, alongside the correct governing standard, removes almost all room for misinterpretation between your team and your supplier.

Frequently Asked Questions

A pipe’s wall thickness is directly related to its pressure rating, strength, weight and flow capacity, which is determined by the pipe schedule. The correct selection of pipe with the same nominal size and outside diameter (OD) can vary in wall thickness according to the schedule to ensure safe and reliable pipe operation. This Pipe Schedule Chart is for the most popular schedules SCH 5, SCH 10, SCH 20, SCH 40, SCH 80, SCH 160 and XXS, and contains standard wall thickness for various pipe sizes. It aids engineers, buyers and fabricators in scheduling, specification checking and choosing the appropriate pipe for industrial applications. Kalpataru Piping offers a variety of carbon steel, stainless steel, alloy steel and special alloy pipes in various schedules to satisfy the requirement of various industries in the world.

Reading the Schedule Chart: What Changes and What Stays Fixed

Reading a schedule chart across this many columns is easier once you know what stays fixed and what moves. Outer diameter never changes across the row, since it is tied to nominal pipe size rather than schedule, which is why a flange rated for a given NPS accepts any schedule without modification. Wall thickness is the only value that climbs from Schedule 5 toward XXS, and inner diameter shrinks with every step up, since bore is outer diameter minus twice the wall. At small sizes, several schedules collapse onto the same thickness, for example Schedule 40 and STD often match, while at larger diameters the schedules spread apart and STD, 40, XS, and 80 report different figures. Available thickness can also vary by manufacturing method, a distinction covered in our difference between ERW and seamless pipe guide.

Pipe Schedule Chart: Wall Thickness in MM for SCH 5, 10, 20, 40, 80, 160 & XXS

Schedule by Schedule: What Each Wall Thickness Class Is Built For

Schedule 5

Schedule 5, sometimes shown as 5S on stainless tables, is the thinnest standardized wall in general circulation, built almost entirely for low pressure and structural service rather than a rated pressure line. At NPS 4, Schedule 5 runs about 2.11 mm, barely a third of the Schedule 40 thickness, which is why it saves considerable weight and cost on runs that only need to contain a fluid at modest pressure or provide a conduit. Because the wall is so thin, Schedule 5 pipe is rarely threaded, since cutting threads into a thin wall removes too great a share of the remaining material and leaves almost nothing to resist bursting. It shows up most often in low pressure water lines, sprinkler distribution, and stainless steel pipes and tubes where corrosion resistance rather than wall strength is doing the heavy lifting.

Schedule 10

Schedule 10 sits one step up from Schedule 5 and has become the default light duty schedule for stainless steel piping, since ASME B36.19M lists it as 10S with dimensions distinct from the carbon steel Schedule 10 table at larger sizes, a difference worth confirming against the stainless steel pipe dimensions chart before ordering. At NPS 6, Schedule 10 measures roughly 3.40 mm against 7.11 mm for Schedule 40, translating directly into lower material cost and easier handling on long stainless runs. Schedule 10 is common in sanitary process piping, low pressure chemical transfer lines, and HVAC systems where the fluid is not aggressive and pressure stays low. It is rarely specified for carbon steel piping beyond mild service, since the thinner wall leaves little corrosion allowance over the design life.

Schedule 20

Schedule 20 occupies a narrower niche than its neighbors and only appears as a distinct entry at larger nominal sizes, typically NPS 8 and above, where it fills the gap between Schedule 10 and Schedule 30 for moderate pressure service on bigger diameters. At NPS 10, Schedule 20 measures about 6.35 mm, noticeably heavier than Schedule 10 at 4.19 mm but still short of the 9.27 mm figure for Schedule 40. Because it barely exists at small sizes, most buyers only encounter Schedule 20 when specifying larger diameter carbon steel pipe for water transmission, low pressure gas gathering, or structural casing, the kind of sizing decision covered in our carbon steel pipe guide. Schedule 40 would represent more wall thickness and cost than the pressure rating requires, making Schedule 20 a genuine cost saver on large bore projects.

Schedule 40

Schedule 40, together with its near identical STD designation at small and mid range sizes, is the wall thickness most piping professionals picture when nobody specifies otherwise, earning that default status through a broad pressure and temperature envelope. At NPS 6, Schedule 40 measures 7.11 mm, comfortably handling typical plant air, water, and low to moderate pressure service without pushing into the heavier, costlier schedules. Because Schedule 40 and STD converge at nearly every size up through roughly NPS 10, the two labels get used almost interchangeably on purchase orders, though above that range the tables diverge and the distinction matters. Its wide availability, combined with a solid, predictable pressure margin, is why every other schedule tends to get measured against it, a role explained in full in our guide to Schedule 40 steel pipe.

Schedule 80

Schedule 80, paired with the older XS or Extra Strong designation, roughly doubles the wall thickness of Schedule 40 at most sizes and is the natural step up whenever a line runs at higher pressure or needs extra allowance for future corrosion or erosion. At NPS 6, Schedule 80 measures 10.97 mm against 7.11 mm for Schedule 40, a jump that raises both the pressure rating and the weight per meter, a figure best confirmed with a proper steel weight formula rather than estimated by eye. Like Schedule 40 and STD, Schedule 80 and XS match closely at small sizes but separate at larger diameters. Schedule 80 is the common choice for steam lines and threaded connections, particularly on seamless pipe, since the extra wall gives threads enough material to hold pressure safely after machining.

Schedule 160

Schedule 160 sits near the top of the standardized range and is reserved for genuinely demanding pressure and temperature combinations rather than routine piping, since the wall thickness increase over Schedule 80 is substantial at every size where it applies. At NPS 6, Schedule 160 runs 18.26 mm, well over double the Schedule 80 figure, and the resulting bore reduction is significant enough that flow calculations must account for the narrower inside diameter, while the added weight is worth checking against a pipe weight calculator before the order is placed. Schedule 160 typically shows up in high pressure steam service, severe chemical process lines, and select oil and gas applications where the design pressure genuinely demands that much wall, since specifying it without justification adds unnecessary weight, cost, and welding difficulty.

XXS (Double Extra Strong)

XXS, or Double Extra Strong, is the heaviest standardized wall class and predates the modern schedule numbering system, having originated alongside STD and XS before the schedule 5 through 160 series was introduced. Because it developed separately, XXS does not have a schedule number and does not scale predictably against Schedule 160 the way Schedule 80 scales against Schedule 40, so it must be looked up directly rather than estimated. At NPS 6, XXS measures roughly 21.95 mm, thick enough that the bore shrinks dramatically compared to lighter schedules, and at larger diameters XXS is not offered at all since the tables stop listing it beyond a certain size. It remains relevant for extreme pressure, high temperature, or heavily eroding service, most often on heavy wall carbon steel pipes and tubes rather than other alloys.

Final Thoughts

Every schedule from 5 through XXS answers one question: how much wall does this line need, no more and no less, to handle its pressure, temperature, and corrosion allowance safely for its design life. Schedule 5 and 10 belong on light duty and stainless service, Schedule 20 fills a large diameter gap most small bore buyers never see, Schedule 40 remains the dependable default, Schedule 80 steps up for higher pressure and threaded connections, and Schedule 160 and XXS are reserved for genuinely severe conditions where anything lighter would be a real risk. Matching the schedule to the actual pressure calculation, rather than habit or a supplier’s default stock, and rechecking the resulting weight against a stainless steel pipe weight chart on stainless projects, is what keeps a system safe without paying for wall thickness never needed.

Frequently Asked Questions