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