Schedule 40 Pipe Dimensions

What Schedule 40 Actually Specifies

A schedule number maps a nominal size to a wall thickness. It is not a single measurement: schedule 40 is 3.91 mm at DN 50 and 8.18 mm at DN 200. The number stays the same; the wall it refers to does not.

The outside diameter is unaffected. That is the whole point of the system — fittings and flanges register on the outside, so it has to be fixed, and the wall varies inside it. See what nominal pipe size means for why that outside diameter isn’t the number printed on the pipe, and how the pipe schedule system works for the other thirteen designations and how much of the grid they actually cover.

Is Schedule 40 the Same as Standard (STD)?

Through NPS 10, yes — every value is identical. From NPS 12 upward they separate, and the gap widens with every size.

Treating the two as interchangeable above NPS 10 is a common and expensive mistake — at NPS 24 the difference is nearly two-fifths of an inch of wall on a pipe of identical outside diameter. The same applies to XS and schedule 80, which part company from NPS 10 upward. Where STD and schedule 40 stop matching works through both pairs at every nominal size.

Schedule 40 in Other Materials

Schedule 40 is not exclusive to steel. PVC pipe sizes use exactly the same outside diameter series — all 19 comparable sizes match — and the wall thickness figures are the same numbers too. What differs is what those numbers mean: ASTM D1785 publishes a minimum wall for PVC, while ASME B36.10M publishes a nominal wall for steel against which a mill under-tolerance is permitted.

Stainless is closer still. Its schedule 40S wall matches schedule 40 at 17 of the 18 sizes the two series share, and parts company at exactly one: 12 inch, where 40S holds 0.375 in against schedule 40’s 0.406 in. That single exception is the same divergence described above — 40S follows the STD wall where schedule 40 has begun to climb away from it. See stainless steel pipe sizes for the full 5S, 10S, 40S and 80S series.

Schedule 40 Outside the American Diameter Series

The schedule numbers travel. The pipe they describe does not. JIS publishes schedule walls that are essentially the ASME walls, and applies them to the Japanese outside diameters — which differ from the American ones at seventeen of the twenty-two sizes the two series share. The same schedule 40 wall figure therefore sits on a different diameter, which means a different bore, a different mass and a different pressure rating, under a designation that reads identically on a drawing.

DN 65 is the clearest case: 73.0 mm under ASME and 76.3 mm under JIS. Specify schedule 40 against each and you have two different pipes with one name, and nothing in the schedule number discloses it. Europe’s threadable tube sidesteps the collision from the other direction: EN 10255 classes its walls light, medium and heavy rather than by schedule, so there is no shared number to be misread in the first place.

A schedule number, in other words, only means something once the diameter series it sits on has been named. That is the half of the specification most charts leave out, and it is set out size by size in what each pipe standard publishes, and where they disagree.

Schedule 40 Compared With Schedule 80

Schedule 80 is the next wall up in common stock, and the comparison is the clearest demonstration of what a schedule number does. The outside diameter is identical in both columns, at every size. The wall thickens, the bore closes in, and the pipe gets heavier — nothing else moves.

The bore is where the choice bites. Going from schedule 40 to schedule 80 at 2 inch costs 3.3 mm of bore — about 12% of the cross-sectional area, on a pipe that looks identical from the outside and takes the same fittings. That is the trade: pressure capability and mechanical robustness bought with flow area and weight.

What Schedule 40 Does Not Tell You

A schedule number is a dimensional specification and nothing more. It says how thick the wall is. It does not say what the pipe is made of, how it was made, what pressure it will hold or what it may lawfully be used for.

That is not a technicality. The same schedule 40 dimensions are published for carbon steel, alloy steel and — through a separate document — stainless, and a run of schedule 40 in one of those materials will not behave like a run in another. Pressure capability depends on the material, its grade, its temperature and the applicable design code; the wall thickness is only one input to that calculation. Anyone quoting schedule 40 as though it were a pressure rating has skipped several steps.

There is also a tolerance to keep in mind. The wall thickness ASME publishes is a nominal figure, against which a mill under-tolerance of 12.5% is permitted — so a pipe stamped schedule 40 can legitimately be delivered with a wall an eighth thinner than the table says. Design calculations use the minimum wall for that reason, not the published one. The outside diameter, by contrast, is held to roughly ±1%, which is why it is the dimension worth measuring and the one every table on this site is keyed on.

Where a Schedule Number Means Anything

Schedule numbers are an American designation. They are published by ASME B36.10M, and 30 of the 157 countries in our country table work to that series — Canada, Mexico, Brazil, Argentina, Saudi Arabia, the United Arab Emirates and the Philippines among them.

One other series borrows the numbers. JIS, the Japanese series used across 9 countries — Cambodia, Indonesia, Japan, Laos, Philippines, South Korea, Taiwan, Thailand and Vietnam — applies the same schedule numbers to its own outside diameters. That is the trap in this page: a JIS schedule 40 pipe at DN 65 is 76.3 mm across with a 5.2 mm wall, where the ASME one is 73.0 mm across with a 5.16 mm wall. Same designation, different pipe, different bore and a different pressure rating.

The rest of the world does not use schedules at all. EN 10220 (56 countries), the BS 1387 and ISO 65 lineage (46 countries, which designates light, medium and heavy classes instead), GOST (15) and GB/T (1) all specify pipe as an outside diameter and a wall thickness, with no schedule number anywhere in the document. On the numbers in that table, 118 of 157 countries would not recognise “schedule 40” as a local designation at all.

So if you are ordering outside North America, the safe request names the outside diameter and the wall in millimetres rather than the schedule. Which series applies where is set out in pipe standards by country, and the diameters are compared side by side in the pipe standards comparison.

Weight of Schedule 40 Pipe

The mass column above is computed, not copied, from the formula the standard itself publishes: W = (OD − t) × t × C, where C is 10.69 for pounds per foot in inches or 0.02466 for kilograms per metre in millimetres, for carbon steel. Pipe weight per foot covers other materials, which need different constants.

Frequently Asked Questions

What is the wall thickness of schedule 40 pipe?

It depends on the nominal size. Schedule 40 is 0.154 in (3.91 mm) at NPS 2, 0.237 in (6.02 mm) at NPS 4, and 0.322 in (8.18 mm) at NPS 8. The schedule number specifies a wall thickness series, not a single value.

Is schedule 40 the same as STD?

Only through NPS 10. From NPS 12 upward they diverge: STD is fixed at 0.375 in while schedule 40 continues to increase with size. At NPS 24 schedule 40 is 0.688 in and STD is still 0.375 in.

Does schedule 40 change the outside diameter?

No. Outside diameter is fixed by the nominal size. Schedule changes only the wall thickness, and therefore the bore. A 2-inch pipe measures 2.375 in across the outside in every schedule.

Is PVC schedule 40 the same size as steel schedule 40?

The outside diameters are identical across all 19 comparable sizes. The wall thickness figures are also the same numbers, but ASTM publishes them as a minimum wall for PVC while ASME publishes a nominal wall for steel, so the tolerances differ.