What is the outside diameter of EN 10255 DN150 pipe?

165.1 mm. It is commonly published as 168.3 mm, which is the EN 10220 and ASME figure for that size rather than the EN 10255 one. The two standards cover different products — EN 10255 is the threadable water and gas tube series, EN 10220 is the general welded and seamless diameter register — and they do not agree at this size.

Why do so many sites say 168.3 mm?

Because one widely-mirrored reference table back-fills the EN 10220 value into its EN 10255 column, and that table is copied on. The two series agree at most sizes, so a substitution at one size does not look wrong from inside the table. It only shows up when the column is checked against the standard it claims to be.

Is EN 10255 the same as BS 1387?

EN 10255 replaced BS 1387 and DIN 2440, but the older wall thicknesses are still in circulation under the new name. Several stockists publish 2.65, 3.25, 4.05 and 4.85 mm — the legacy walls — under an EN 10255 heading. It is a labelling error rather than a different product, and it is caught by arithmetic rather than by eye.

How can you tell which wall a published table is really using?

From the mass. At DN25 medium, a published 2.41 kg/m computes to 2.407 for a 3.2 mm wall and 2.440 for a 3.25 mm wall. The published figure identifies the 3.2 mm wall, so a table printing 3.25 mm beside that mass is carrying a legacy wall under a current heading.

Does this affect anything other than DN150?

The diameter substitution is specific to DN150, which is precisely why it survives — every neighbouring size agrees, so the column looks internally consistent. The wall-thickness mislabelling is separate and runs across several sizes.

EN 10255 DN150 Is 165.1 mm, Not 168.3

The figure most reference tables print in this cell belongs to a different standard. EN 10255 — the threadable water and gas tube series — puts DN150 at 165.1 mm, not the 168.3 mm of EN 10220 and ASME. A 3.2 mm error, in the one cell where nothing else in the column disagrees.

What This Claim Rests On

EN 10255 is not one of our compiled datasets. The 9892 wall rows behind this site cover ASME, EN 10220, JIS, GOST and GB/T, and EN 10255 is not among them — so 165.1 mm is not a row we hold with our own verification against it. It comes from the compilation record: five independent publishers, including a manufacturer’s own datasheet, plus a separate confirmation arriving from the Russian side of the research, against one widely-mirrored table giving 168.3. We think that is decisive. We are telling you what it is rather than presenting it as something we measured, and this page carries no Dataset markup for the same reason.

The figures for EN 10220 and ASME on this page are ours, read from the compiled data — see how this data was verified.

Two Standards, One Cell

EN 10255 and EN 10220 are not competing versions of one table. They describe different products. EN 10255 covers the threadable tube traditionally used for water and gas, the descendant of BS 1387 and DIN 2440. EN 10220 is the general outside diameter register for welded and seamless steel pipe. A tube can only be made to one of them.

DN150 outside diameter by standard. Only the EN 10220 and ASME rows are read from our compiled data; the EN 10255 figure is sourced as described above.
Standard	What it covers	DN150 OD (mm)
EN 10255	Threadable water and gas tube	165.1
EN 10220	General welded and seamless diameter register	168.3
ASME B36.10M-2015	American steel pipe	168.3

EN 10220 and ASME agree at this size, which is exactly why the substitution survives. Drop the wrong number into the EN 10255 column and it still matches both of its neighbours in the reference table. Nothing looks wrong until the column is checked against the standard it is labelled with.

The Consequence: It Lines Up With Russia Instead

The correct figure has a second-order effect that the wrong one hides. At 165.1 mm, EN 10255 sits alongside the Russian water and gas tube series rather than the American one — the two traditions meet here, at the size where the European general register does not. Printing 168.3 in this cell erases a real relationship between two standards and invents one that does not exist.

The wider comparison between those registers is at GOST and the European series. For the general European diameter table, see the EN 10220 outside diameter series.

The Related Error: Legacy Walls Under a Current Name

The same pass found a second problem in EN 10255 tables, and this one is about wall thickness rather than diameter. Several UK stockists publish the old BS 1387:1985 and DIN 2440 walls — 2.65, 3.25, 4.05 and 4.85 mm — under an EN 10255 heading. The heading is current, the numbers are not.

Reading the table cannot catch that, because both sets of walls look equally plausible. Arithmetic can. At DN25 medium the published mass is 2.41 kg/m, and the standard’s own mass formula, M = (OD - t) * t * C, tells you which wall that belongs to:

3.2 mm — EN 10255 medium: (33.7 − 3.2) × 3.2 × 0.02466 = 2.407 kg/m — matches the published 2.41
3.25 mm — BS 1387:1985 / DIN 2440 medium: (33.7 − 3.25) × 3.25 × 0.02466 = 2.440 kg/m — does not match

So the published mass is describing the 3.2 mm wall. A table printing 3.25 mm next to it has taken the thickness from one edition and the mass from another.

One caveat, in the interest of not overstating the method: EN 10255 computes mass from the mean of the outside diameter tolerance band rather than from the nominal diameter, so the figures above are close approximations rather than the standard’s own arithmetic. They are precise enough to tell two candidate walls apart, which is all that is being asked of them here.

What To Do With This

If you are specifying or checking threadable tube at DN150, take the diameter from EN 10255 itself rather than from a general reference table, and treat any EN 10255 column that prints 168.3 mm at this size as carrying at least one substituted figure.

Galvanized tube in Europe is normally this product, so the same caution applies there — see galvanized pipe sizes. Every error we have found in published tables, including our own, is listed at errors found in published pipe data.

Frequently Asked Questions

What is the outside diameter of EN 10255 DN150 pipe?: 165.1 mm. It is commonly published as 168.3 mm, which is the EN 10220 and ASME figure for that size rather than the EN 10255 one. The two standards cover different products — EN 10255 is the threadable water and gas tube series, EN 10220 is the general welded and seamless diameter register — and they do not agree at this size.
Why do so many sites say 168.3 mm?: Because one widely-mirrored reference table back-fills the EN 10220 value into its EN 10255 column, and that table is copied on. The two series agree at most sizes, so a substitution at one size does not look wrong from inside the table. It only shows up when the column is checked against the standard it claims to be.
Is EN 10255 the same as BS 1387?: EN 10255 replaced BS 1387 and DIN 2440, but the older wall thicknesses are still in circulation under the new name. Several stockists publish 2.65, 3.25, 4.05 and 4.85 mm — the legacy walls — under an EN 10255 heading. It is a labelling error rather than a different product, and it is caught by arithmetic rather than by eye.
How can you tell which wall a published table is really using?: From the mass. At DN25 medium, a published 2.41 kg/m computes to 2.407 for a 3.2 mm wall and 2.440 for a 3.25 mm wall. The published figure identifies the 3.2 mm wall, so a table printing 3.25 mm beside that mass is carrying a legacy wall under a current heading.
Does this affect anything other than DN150?: The diameter substitution is specific to DN150, which is precisely why it survives — every neighbouring size agrees, so the column looks internally consistent. The wall-thickness mislabelling is separate and runs across several sizes.