Nominal Pipe Size Explained

Why 2-Inch Pipe Isn’t 2 Inches

The designation is a survival from the nineteenth century, when pipe was wrought iron and the number approximated the bore — the hole down the middle. Walls were thick and inconsistent, so the inside was what people cared about.

As manufacturing improved, wall thicknesses came down. But threads, couplings and flanges all register on the outside of the pipe, so the outside diameter had to be frozen or nothing would fit anything else. It was frozen. The wall kept changing, the bore changed with it, and the name — which had described the bore — stopped describing anything at all.

That is why the number on the pipe and the number on your calliper disagree, and why identifying a pipe needs the full pipe size chart rather than arithmetic.

Nominal Size vs Outside Diameter

Showing ASME B36.10M-2015. 766 of 772 rows verified against the standard itself; every figure is checked against the standard’s own mass formula. How this data was verified.

Where the Numbers Start Matching

From NPS 14 upward, the nominal size and the outside diameter in inches are the same number. NPS 14 pipe is exactly 14.000 inches across the outside; NPS 24 is exactly 24.000 inches. Below that, the gap is proportionally largest at the small end — ⅛-inch pipe measures 0.405 in, more than three times its nominal size.

How Wall Thickness Fits In

Nominal size fixes the outside diameter. A schedule number then fixes the wall. The inside diameter is simply what is left over:

ID = OD − (2 × wall)

This is why the bore of a pipe is never a round number, and why “2-inch schedule 80” has a smaller opening than “2-inch schedule 40” despite both being 2-inch pipe.

So Is the Name At Least the Bore?

It is the obvious rescue for the nominal system: the name never matched the outside, but it began life as an approximation of the bore, so perhaps it still describes the hole. Set the two against each other and the answer is a qualified no. The bore is not a single number to compare against — it moves with every schedule — and the name lands somewhere inside that range at every size while matching no particular wall.

Read the figure carefully and it says something more interesting than “the name is wrong”. The name is a decent description of the bore of a moderately thick-walled pipe, which is what the designation approximated when it was set in the nineteenth century. What has happened since is that walls got thinner and more numerous: the schedule system spread the available bores out around the old value, so the name now sits in the middle of a range rather than on a pipe. That is precisely what “nominal” means, and it is the reason the outside diameter — the one dimension that never moved — became the identifier the standards are keyed on.

NPS and DN Are Not the Same System

DN is the metric designation. Above DN 100 it behaves arithmetically — DN is 25 times the NPS. Below DN 100 it does not, and the mapping is an irregular lookup you simply have to know:

There is no formula that produces 6, 8, 10, 15, 20, 25, 32, 40, 50, 65, 80 from ⅛ through 3. That table is the reason a conversion chart exists at all — see NPS to DN conversion.

What DN Actually Measures

Here is the part almost nobody states. ISO 6708, the standard that defines DN, describes it as 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. The standard never chose, and a note instructs each downstream standard to declare its own relationship.

That single unresolved definition is why DN 65 is 73.0 mm under ASME, 76.1 mm under EN, and 76.3 mm under JIS. The systems didn’t drift apart — they were never tied together in the first place.

Standards That Name Pipe Without a Nominal Size

Not every system has one. GOST 8732, 8734 and 10704 — the Russian and CIS seamless and welded families — designate a pipe by outside diameter and wall thickness and nothing else. The clause is explicit that pipe is made to the outside diameter, the wall thickness and the length, and an order reads as a diameter, a wall and a length against the standard number: 70 by 3.5 by 1250 is a 70 mm tube with a 3.5 mm wall. No nominal number appears in it anywhere. GB/T 17395, the Chinese diameter standard, is likewise a flat list of permitted outside diameters with no DN column at all.

That has a consequence worth stating plainly. The DN figures circulated for both are trade convention rather than standards text. Independent sources agree on each mapping, and neither document publishes one, so printing them as “per GOST” or “per GB/T” would claim an authority the standard does not carry. It is the reason the tables here for those two are keyed on the outside diameter you can actually measure, and it is a useful corrective to the idea that a nominal size is a universal feature of pipe rather than an inherited convention of one tradition. See what each pipe standard publishes.

Where the Nominal Number Still Means the Bore

One series inverts the rule, and it shows what the others gave up. GOST 3262, the Russian threadable water and gas standard, designates pipe by its Ду — its nominal bore — so the number in the designation still points at the hole rather than at the outside. A GOST 3262 pipe called 20 has an outside diameter of 26.8 mm, and the 20 approximates what is left inside once the wall is taken off.

Set that beside NPS ¾, whose outside diameter is 26.7 mm, and the two are the same physical tube to within a tenth of a millimetre. One of them is named after its bore and one is named after nothing at all — which is the whole history of the nominal system in a single pair of numbers.

Nominal Bore, NB and the Other Names

NPS

The North American designation, in inches. Used by ASME and ASTM.

DN

The metric designation, defined by ISO 6708. Dimensionless.

NB

Used in India, Australia and much of the Commonwealth. Numerically identical to DN.

IPS

An older North American term, still used for plastic pipe sized to the steel series.

Frequently Asked Questions

What does nominal pipe size mean?

Nominal pipe size is a name, not a measurement. It is a trade designation that identifies a pipe's outside diameter without stating it. A 2-inch pipe measures 2.375 inches across the outside.

Why is nominal pipe size different from the actual size?

The designation originally approximated the bore of wrought iron pipe in the nineteenth century. As manufacturing improved, wall thicknesses changed but the outside diameter had to stay fixed so fittings would still fit. The name was kept and stopped describing anything measurable.

At what size does NPS equal the outside diameter?

From NPS 14 upward. NPS 14 pipe has an outside diameter of exactly 14.000 inches, and every larger size follows the same rule. Below NPS 14 the nominal size is always smaller than the outside diameter.

Is DN the same as NPS?

They name the same physical pipe but they are separate systems. Above DN 100 the relationship is arithmetic — DN is 25 times NPS. Below DN 100 it is an irregular lookup that no formula reproduces.

What is nominal bore?

Nominal bore, or NB, is the term used in India, Australia and much of the Commonwealth for the same designation that DN describes. NB 50, DN 50 and NPS 2 all refer to the same pipe.