Stainless Steel Pipe Sizes

What the S in 40S and 80S Actually Means

The S is not a decoration on the carbon schedule number. Checking our own tables against each other, the 40S and 80S walls line up with the older STD and XS designations rather than with schedules 40 and 80 — at every size the stainless standard covers.

That distinction is invisible until it bites. STD and schedule 40 are the same value through NPS 10 and separate above it, so at NPS 12 the 40S wall is 0.375 in while schedule 40 is 0.406 in on a pipe of identical outside diameter. Anyone substituting one for the other above NPS 10 is changing the wall by 0.031 in without changing anything visible from the outside. Schedule 40 pipe dimensions sets out where that divergence starts.

5S and 10S have no STD or XS counterpart at any size. They are the thin-wall end of the stainless series, and 5S is not published at the three smallest sizes at all — the standard starts it at NPS ½.

What Stainless Shares With the Other Materials

The outside diameter series is common ground. Every one of the 69 outside diameters here, in inches, also appears in ASME B36.10M-2015, which means a stainless pipe and a carbon pipe of the same nominal size take the same fittings, flanges and threads. The bore does not match, because the walls differ — but the outside does.

Carbon steel pipe sizes

ASME B36.10M-2015 — the full schedule series, verified

Galvanized pipe sizes

the carbon steel dimensions with a zinc coating over them

A stainless pipe cannot be told from a carbon one with a tape measure, for exactly this reason. How to measure pipe size gets you the nominal size; it will not get you the material or the schedule. And if the number stamped on the pipe is what is confusing you, start with what nominal pipe size actually means.

The S Schedules Are American Too

5S, 10S, 40S and 80S are designations of ASME B36.19M, an ASME document, and they sit on the ASME outside-diameter series — the same diameters as carbon steel to ASME B36.10M-2015. Both halves of that are American, which matters the moment the pipe is bought somewhere else.

30 of the 157 countries in our country table default to the ASME diameter series — Canada, Mexico, Brazil, Saudi Arabia and the United Arab Emirates among them — so the figures on this page are the ones in force there. Elsewhere the diameter underneath the wall changes: EN 10220 across 56 countries, the BS 1387 and ISO 65 lineage across 46, JIS across 9 including Japan, South Korea and Taiwan, and GOST across 15. At DN 65 that is 73.0 mm under ASME against 76.1 mm under EN and 76.3 under JIS — three different pipes under one size name, and a fitting bored for one will not seat on another whatever it is made of.

None of those series publishes an S-schedule. Where they specify stainless they do it as an outside diameter and a wall thickness directly, so a request for “40S” outside the ASME countries has to be translated into millimetres before it means anything. The diameters are compared size by size in the pipe standards comparison.

What We Could Not Verify

One publisher carries this table, and we have not matched it against a second. That is a genuinely weaker footing than the carbon steel pages, where 766 of 772 rows were checked against ASME B36.10M-2015 itself. Source count is not source independence, so finding another site with the same numbers would not close it either — several apparently separate publishers turned out to be reproducing one another during compilation.

The open items on this table, stated rather than hidden: no second source for ASME B36.19M; no verified density for austenitic stainless, so no stainless mass constant; and no independent confirmation of the 5S, 10S, 40S and 80S assignment, only of the numbers themselves.

What that means in practice is worth being exact about, because “unverified” is not the same as “probably wrong”. The diameters on this page agree with the carbon steel series they are shared with, and that agreement is itself a check the numbers pass. What has not been done is the check that carries the carbon steel tables: reading the dimension table in the standard document and comparing it value by value. Until that happens the figures here are a good working reference and not a citable fact, and the page says so rather than letting the distinction quietly disappear.

Frequently Asked Questions

Is stainless pipe the same size as carbon steel pipe?

The outside diameters are identical. All 69 rows in ASME B36.19M carry an outside diameter in inches that ASME B36.10M-2015 also publishes at the same nominal size. The wall series is what differs: stainless adds the thin 5S and 10S walls, and its 40S and 80S walls follow STD and XS rather than schedules 40 and 80.

What does the S mean in schedule 40S?

It marks the wall as belonging to the stainless series in ASME B36.19M rather than the carbon series in ASME B36.10M. Across all 18 nominal sizes the stainless standard covers, 40S is the STD wall and 80S is the XS wall.

Is schedule 40S the same as schedule 40?

Not everywhere. Through NPS 10 the STD and schedule 40 walls agree, so 40S matches schedule 40 there. At NPS 12 they part: 40S is 0.375 in, which is the STD value, while schedule 40 is 0.406 in.

How much heavier is stainless pipe than carbon steel pipe?

We do not publish a figure. The mass columns in the table are computed with the carbon steel constant, 0.02466 kg/m. The publisher we compiled the table from states austenitic stainless is 8.0 g/cm3 against 7.85 for carbon steel, which would put the true figures roughly 2% higher, but we have not confirmed that density against a standard and so publish no corrected constant.