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

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