Steel Pipe Size Chart: NPS, DN, OD, Wall Thickness Guide

A two inch pipe does not measure two inches across. Its outside diameter is 60.3 mm, and it has been that number for longer than most of the plants using it have been standing. Your tape measure is fine. Nominal Pipe Size is a designator, not a dimension, and that single fact is behind most of the misreadings of a steel pipe size chart that happen somewhere between a drawing and a purchase order. Below are the numbers, in NPS and DN, with the outside diameters that never change and the wall thicknesses that do, for carbon steel and for stainless steel pipe. Then the part most charts leave out. The carbon steel schedules and the stainless S schedules are not the same numbers, and above NPS 10 they are not close.

Why NPS is not the outside diameter

When pipe was first standardised, the nominal number was a rough description of the bore. Wall thicknesses were what they were, and the outside diameter followed.

Then the industry did the sensible thing and standardised the outside diameter instead, because threading dies, flanges and fittings all key off the OD. The bore became whatever was left after the wall was subtracted. The nominal name stayed.

So NPS 2 has an OD of 60.3 mm regardless of whether the wall is 2.77 mm or 11.07 mm. The bore changes. The OD does not. This is the entire logic of the schedule system, and once it is clear, every chart on this page reads itself.

Two more points that catch people out:

• From NPS 14 upward, NPS does equal the OD in inches. NPS 14 is 14.000 in, NPS 24 is 24.000 in. The odd behaviour is confined to NPS 1/8 through 12.
• DN is not a conversion of NPS. DN 50 is the ISO designation that corresponds to NPS 2, but 50 is not 60.3 and it is not 2 inches in millimetres either. It is a label. Use the chart, do not do the arithmetic.

Steel pipe size chart: NPS, DN and outside diameter

The outside diameter for a given size is fixed for both carbon steel and stainless. Only the wall changes.

These charts are for estimating and enquiry purposes. Dimensional values used for design or fabrication must be taken from the current edition of the governing standard and from the mill test certificate. Metric figures below are the standard’s own listed millimetre values, not conversions, so they will not always match a unit calculator exactly.

What a schedule actually specifies

A schedule number specifies wall thickness. It does not specify a constant wall thickness.

SCH 40 at NPS 1/2 is 2.77 mm. SCH 40 at NPS 24 is 17.48 mm. Same schedule, six times the wall. The schedule is a series designed to hold roughly similar pressure capability across the size range, not a thickness in its own right, which is why “we always use schedule 40” is a specification habit rather than a specification.

The higher the schedule number, the thicker the wall and the smaller the bore. The OD stays where it is.

Carbon and alloy steel pipe size chart (ASME B36.10M)

Wall thickness in mm, with inches in brackets. This is the table that governs carbon steel and alloy steel pipe, including the common ASTM A106, A53 and A335 grades.

NPS 22 has no SCH 40 listing. That is not a gap in the table, it is how the standard is written.

Stainless steel pipe size chart (ASME B36.19M)

Stainless, duplex and nickel alloy pipe uses the S schedules. Same outside diameters, different wall series, and a much shorter list of available thicknesses.

These are the wall thicknesses that apply to SS 304 pipe, SS 310, SS 317L and SS 904L.

Where the S schedules stop agreeing with the plain schedules

This is the section most pipe size charts leave out, and it is the one that costs money.

Below NPS 10, SCH 40S and SCH 40 are the same number, and so are 80S and 80. People learn that, generalise it, and then order a large-bore line on the assumption it still holds. It does not.

At NPS 24, SCH 80 carbon steel is 30.96 mm and SCH 80S stainless is 12.70 mm. That is less than half the wall. Above NPS 12, the S schedules simply stop increasing: 40S is 9.53 mm and 80S is 12.70 mm all the way up, because they were never intended as pressure classes at those diameters.

There is a matching trap at the small end. At NPS 3/8, SCH 10 is 1.85 mm and SCH 10S is 1.65 mm.

The practical rule: write the wall thickness on the enquiry, not just the schedule. A drawing that says “NPS 16, SCH 40” is unambiguous only if it also says which material and therefore which standard applies. The same applies when a line specification mixes carbon steel headers with duplex 2205 or super duplex branches.

STD, XS and XXS, and why XXS is not always the thickest

Before schedule numbers existed there were three weights: Standard, Extra Strong and Double Extra Strong. They are still in the tables and still in daily use.

• STD matches SCH 40 up to and including NPS 10. From NPS 12 upward, STD is fixed at 9.53 mm (0.375 in) whatever the diameter.
• XS matches SCH 80 up to and including NPS 8. From NPS 10 upward, XS is fixed at 12.70 mm (0.500 in).
• XXS follows no schedule at all.

That last one produces a result that looks wrong and is not:

At NPS 8 and above, SCH 160 is thicker than XXS. The name is historical, not a ranking. If someone specifies XXS expecting the heaviest available wall at 12 inch, they are 7.92 mm short of it.

How to work out the inside diameter

Inside diameter is not specified in the standard. It is calculated, and it carries the tolerance of everything used to calculate it.

ID = OD − (2 × wall thickness)

NPS 4, SCH 40:

• OD = 114.30 mm (4.500 in)
• Wall = 6.02 mm (0.237 in)
• ID = 114.30 − 12.04 = 102.26 mm (4.026 in)

Same size in SCH 80:

• Wall = 8.56 mm (0.337 in)
• ID = 114.30 − 17.12 = 97.18 mm (3.826 in)

Two schedules, identical outside diameter, 5 mm of bore difference. For a flow calculation that matters, because area scales with the square of the bore.

Once you have OD and wall, weight follows from the same two numbers. Our steel pipe weight chart and calculator gives the formula, a worked example and the weight per metre for every schedule below.

The chart is nominal, the pipe is not

Worth saying plainly, because it is the difference between a chart and a delivery.

Wall thickness carries a manufacturing tolerance of minus 12.5 per cent on the nominal figure. An 8.18 mm nominal wall can legitimately arrive at 7.16 mm and be fully compliant with the standard and its certificate. Outside diameter carries its own tolerance, tighter on small bore and proportional on large bore.

So:

• Design to the minimum wall, not the nominal wall, wherever the calculation is tight.
• Do not use a chart figure to check whether delivered material is correct. Use the mill test certificate and measure.
• If a project requires the nominal wall as a minimum, that has to be stated at enquiry stage. It is a purchasing condition, not something the standard gives you by default.

Sending an enquiry that cannot be misread

Four things remove almost all ambiguity:

1. Material and standard. “ASTM A312 TP316L” tells a supplier which dimensional standard applies. “Stainless” does not.
2. NPS or DN, and say which. DN 300 and NPS 300 are not the same request.
3. Wall thickness in mm, with the schedule in brackets. Schedule alone is ambiguous across standards above NPS 10.
4. Length and end finish. Random, double random or fixed, plain end or bevelled.

Send us the OD, wall, schedule and length and we will confirm availability against current stock and mill programmes. Our full pipe and tube range covers stainless, duplex, super duplex, nickel alloy and titanium.

Two related decisions sit alongside the size itself. Which specification governs the material is covered in ASTM A312 vs A269 vs A213, and whether to buy seamless or welded is covered in seamless vs welded steel pipe. Both change what the same nominal size actually costs to deliver.