Is API 5L pipe the same size as ASME B36.10M pipe?

On the outside, almost exactly. Across the 44 nominal sizes both documents carry, they print the same outside diameter at 42 of them. The 2 exceptions — NPS 10 and 12 — differ by 0.1 mm, which is a difference in how the number was rounded rather than in the pipe.

Why does API 5L say 273.1 mm where ASME says 273.0?

Because the exact conversion is a dead heat. NPS 10 is 10.75 inches, and 10.75 × 25.4 is exactly 273.05 mm — precisely half way between 273.0 and 273.1. There is no arithmetic answer to which way that should go, so each document made its own choice and they chose opposite ways.

Do API 5L and ASME B36.10M use the same wall thicknesses?

Compared at the one decimal place API publishes, 572 of API 5L's walls also appear in ASME B36.10M, against 2 that do not. ASME carries a further 194 that API does not list. So API's wall series is very nearly a subset of ASME's rather than a rival to it — line pipe does not carry the full schedule ladder.

Which standard should a drawing name?

Whichever governs the product. API 5L is line pipe, written around grade and test requirements for pipelines; ASME B36.10M is the general dimensional standard. They are not two answers to one question, and on dimensions they very nearly agree — so the choice is about scope, not about which numbers are right.

Is the 0.1 mm difference enough to matter?

Not for fit. It is a tenth of a millimetre on a pipe over 270 mm across, well inside any manufacturing tolerance, and both documents describe the same physical product. It matters as a specification question: two documents printing two numbers for one pipe is worth knowing about before someone reconciles a datasheet against a drawing and thinks one of them is wrong.

API 5L vs ASME B36.10M

Two documents over one dimensional series. Across the 44 nominal sizes both carry, they print the same OD at 42. The 2 that differ do so by 0.1 mm — and at exactly the two sizes where the inch-to-millimetre conversion is a dead tie.

The Two Diameters That Differ

Every nominal size where API 5L and ASME B36.10M-2015 print a different outside diameter, with the exact conversion each was rounded from.
NPS	OD (in)	Exact (mm)	ASME prints	API prints
10	10.750	273.05	273.0	273.1
12	12.750	323.85	323.8	323.9

Look at the exact column. Both land on a 5 in the second decimal — precisely half way between the two figures either side of them. That is the whole disagreement.

A Dead Tie, Broken Two Ways

An inch diameter converts to millimetres by multiplying by 25.4, and for most sizes the answer is nowhere near a boundary. NPS 6 is 6.625 in, which is 168.275 mm — it rounds to one decimal place without anyone having to decide anything.

At NPS 10 and NPS 12 the arithmetic stops helping. 10.75 × 25.4 = 273.05, and 12.75 × 25.4 = 323.85. Each sits exactly on the half-way mark, and rounding rules have to invent an answer where the arithmetic declines to give one. ASME B36.10M-2015 rounds down; API 5L rounds up. Neither is wrong.

These are the only two sizes in the whole shared range where the conversion is a tie, which is why they are the only two sizes where the documents disagree. The correspondence is exact, and this page fails to build if a divergence ever appears that is not a tie.

The Walls Are Nearly a Subset, Not a Rival Series

Agreeing on the outside is not the same as being the same table, so the wall thicknesses are worth checking separately. Compared at the one decimal place API publishes, 572 of its walls also appear in ASME B36.10M-2015, against 2 that do not. ASME carries a further 194 walls that API does not list at all.

So API 5L is very nearly a subset. That is what you would expect from what it is: a line pipe specification, concerned with grade, testing and pressure containment, which needs a working range of wall thicknesses rather than the full schedule ladder. The place to see the ladder in full is steel pipe sizes, and how the pipe schedule system works covers what the schedule number is doing.

Showing API 5L 43rd ed (2004). 574 of 592 parsed rows verified against M = (D - t) * t * 0.0246615 kg/m, API 5L cl. 9.11.2 and against ID = D − 2t; 18 are excluded as parse damage rather than published with a caveat, because a crossed column is not a reading of the standard at all. How this data was verified.

Where They Agree, Which Is Nearly Everywhere

The 42 sizes both documents print identically run from NPS ⅛ to NPS 80. That is the useful headline, and it is the opposite of the one a reader usually arrives with: the interesting thing about API 5L and ASME B36.10M is not that they differ, it is that two standards written for different purposes by different committees land on the same 42 diameters to the last decimal.

Compare that with the genuine disagreements elsewhere. At DN65 the American and European series are millimetres apart, which is a different pipe rather than a different rounding — why 2½-inch pipe has three different diameters. And the Russian series carries rounded integers where Europe carries decimals, in GOST and the European series. Against those, 0.1 mm at two sizes is agreement.

Frequently Asked Questions

Is API 5L pipe the same size as ASME B36.10M pipe?: On the outside, almost exactly. Across the 44 nominal sizes both documents carry, they print the same outside diameter at 42 of them. The 2 exceptions — NPS 10 and 12 — differ by 0.1 mm, which is a difference in how the number was rounded rather than in the pipe.
Why does API 5L say 273.1 mm where ASME says 273.0?: Because the exact conversion is a dead heat. NPS 10 is 10.75 inches, and 10.75 × 25.4 is exactly 273.05 mm — precisely half way between 273.0 and 273.1. There is no arithmetic answer to which way that should go, so each document made its own choice and they chose opposite ways.
Do API 5L and ASME B36.10M use the same wall thicknesses?: Compared at the one decimal place API publishes, 572 of API 5L's walls also appear in ASME B36.10M, against 2 that do not. ASME carries a further 194 that API does not list. So API's wall series is very nearly a subset of ASME's rather than a rival to it — line pipe does not carry the full schedule ladder.
Which standard should a drawing name?: Whichever governs the product. API 5L is line pipe, written around grade and test requirements for pipelines; ASME B36.10M is the general dimensional standard. They are not two answers to one question, and on dimensions they very nearly agree — so the choice is about scope, not about which numbers are right.
Is the 0.1 mm difference enough to matter?: Not for fit. It is a tenth of a millimetre on a pipe over 270 mm across, well inside any manufacturing tolerance, and both documents describe the same physical product. It matters as a specification question: two documents printing two numbers for one pipe is worth knowing about before someone reconciles a datasheet against a drawing and thinks one of them is wrong.