What does DN mean on a pipe?

DN is Diamètre Nominal, the metric nominal size designation. It is a label that identifies a size within a standard — it is not a measurement of the pipe. DN50 pipe is neither 50 mm across the outside nor reliably 50 mm across the bore, and which physical diameter it names depends on which standard is being used.

Is DN the outside diameter or the inside diameter?

Neither, as a rule. It is a name for a position in a size series. Historically the number approximated the bore, but wall thicknesses changed while the outside diameter was standardised, and the label stayed where it was. The same is true of the inch designation — see nominal pipe size for how that came about.

Does DN50 mean the same thing in every country?

Not exactly. Across the 7 sizes compared here the three bore-designated series agree at 1 and disagree at 6, by between 0.2 and 3.3 mm. Most of those gaps are small enough to matter only as a specification question. The one at DN65 is not: at 3.3 mm nothing that locates on the outside diameter will interchange.

How do I convert DN to an outside diameter?

Only by naming the standard first. There is no universal DN-to-OD formula, and the conversion tables published without a standard against them are trade convention rather than standards text. Two of the largest systems in the world — GOST and GB/T — have no DN column in the standard to convert from at all.

What is the difference between DN and NPS?

They are two naming systems for the same physical size series, one metric and one imperial. Above DN 100 the relationship is arithmetic, DN equals 25 times NPS; below it the pairing is an irregular lookup that no formula reproduces.

What DN Actually Measures

DN is a label, not a dimension. It identifies a position in a size series, and which physical pipe that position corresponds to depends entirely on whose series you are reading. Across 7 nominal sizes, the three bore-designated standards here disagree at 6 of them.

The Same DN, Three Different Diameters

Outside diameter in millimetres at each nominal size, read from 9892 compiled wall rows. A blank means the series lists no diameter in that range.
DN	NPS	ASME B36.10M	EN 10220	JIS G3452	Spread
50	2	60.3	60.3	60.5	0.2 mm
65	2½	73.0	76.1	76.3	3.3 mm
80	3	88.9	88.9	89.1	0.2 mm
100	4	114.3	114.3	114.3	agree
125	5	141.3	139.7	139.8	1.6 mm
150	6	168.3	168.3	165.2	3.1 mm
200	8	219.1	219.1	216.3	2.8 mm

They agree at 1 of 7 sizes and part company at 6. The worst is DN65, at 3.3 mm — enough that nothing locating on the outside diameter will interchange. That size has a page of its own: why 2½-inch pipe has three different diameters.

Why the Number Is Not a Measurement

The nominal number began as an approximation of the bore, back when wall thicknesses were heavier and more uniform than they are now. Two things then happened to it. Walls got thinner and more varied, so the bore moved away from the label; and the outside diameter was standardised, because that is the surface threads, flanges and fittings have to locate on. The name stayed where it started.

The standard that defines DN says as much. ISO 6708 calls it an alphanumeric designation of size … which is indirectly related to the physical size, in millimetres, of the bore or outside diameter of the end connections. Bore or outside diameter — it never chose between them, and a note instructs each downstream standard to declare its own relationship. The disagreement in the table above is not the standards drifting apart. It is the definition declining to tie them together, and each one then answering for itself exactly as instructed.

So DN50 is not 50 mm across anything in particular. It is the name of a size at which one standard says 60.3 mm and another says 60.5 mm. The inch designation has the same history and the same gap between the name and the object — nominal pipe size explained works through it, and how to measure pipe size covers the reverse problem of identifying a pipe you already have.

Two Major Systems Have No DN Column At All

The comparison above is only possible because those three standards designate pipe by nominal bore. Not everyone does. GOST and GB/T designate a pipe as outside diameter and wall thickness directly — a Russian designation names the two numbers and nothing else. There is no DN column in the standard to look anything up in.

GOST 8732-2025: Russia and the CIS. A flat list of 70 permitted outside diameters, with no nominal-bore designation against them. See GOST pipe dimensions.
GB/T 17395-2008: China. A flat list of 100 permitted outside diameters, with no nominal-bore designation against them. See the GB/T 17395 Chinese outside diameter series.

This matters more than it sounds. DN-to-OD conversion tables for these systems circulate widely and they are trade convention, not standards text — the standard does not make the claim the table attributes to it. We hold those mappings, and we publish them as convention rather than as the standard’s own words, because the difference is the difference between a fact and a custom.

What To Do With This

Treat DN as half a specification. It tells you which row of a table to read, and it does not tell you which table. A drawing calling for DN65 without naming a standard is under-specified by 3.3 mm, and the error surfaces at the joint rather than on delivery.

For the full picture of what each document publishes, see pipe standards compared and pipe standards by country. The individual series are at ASME B36.10M steel pipe dimensions, the EN 10220 outside diameter series and JIS G3452 Japanese pipe dimensions. To move between the metric and imperial names of the same size, use the NPS to DN conversion table.

Frequently Asked Questions

What does DN mean on a pipe?: DN is Diamètre Nominal, the metric nominal size designation. It is a label that identifies a size within a standard — it is not a measurement of the pipe. DN50 pipe is neither 50 mm across the outside nor reliably 50 mm across the bore, and which physical diameter it names depends on which standard is being used.
Is DN the outside diameter or the inside diameter?: Neither, as a rule. It is a name for a position in a size series. Historically the number approximated the bore, but wall thicknesses changed while the outside diameter was standardised, and the label stayed where it was. The same is true of the inch designation — see nominal pipe size for how that came about.
Does DN50 mean the same thing in every country?: Not exactly. Across the 7 sizes compared here the three bore-designated series agree at 1 and disagree at 6, by between 0.2 and 3.3 mm. Most of those gaps are small enough to matter only as a specification question. The one at DN65 is not: at 3.3 mm nothing that locates on the outside diameter will interchange.
How do I convert DN to an outside diameter?: Only by naming the standard first. There is no universal DN-to-OD formula, and the conversion tables published without a standard against them are trade convention rather than standards text. Two of the largest systems in the world — GOST and GB/T — have no DN column in the standard to convert from at all.
What is the difference between DN and NPS?: They are two naming systems for the same physical size series, one metric and one imperial. Above DN 100 the relationship is arithmetic, DN equals 25 times NPS; below it the pairing is an irregular lookup that no formula reproduces.