NPS to DN Conversion

The Complete NPS to DN Conversion Table

All 44 nominal sizes in ASME B36.10M-2015, with the outside diameter both designations refer to. The third column is the arithmetic — wherever it disagrees with the DN beside it, the arithmetic is what is wrong.

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.

Above DN 100 the Conversion Is Arithmetic

For all 32 sizes from NPS 4 to NPS 80, DN is exactly 25 times the nominal size. NPS 12 is DN 300; NPS 18 is DN 450; DN 600 is NPS 24. There are no exceptions in the standard, in either direction.

DN = 25 × NPS   (DN 100 and above)

What that does not mean is that DN is a measurement in millimetres. DN 300 pipe is not 300 mm across anything: its outside diameter is 323.8 mm, and its ID depends on which schedule it was made to. The 25 is a convention that converts one label into another label, not inches into millimetres.

The rule is worth stating with its boundary attached, because it is so often given without one. “DN equals 25 times NPS” is repeated chart after chart as though it held throughout, and it is exact from DN 100 up and wrong at nearly every size below. Plotted against the sizes the standard actually publishes, the error is not a rounding wobble — it walks up and down by several millimetres, then stops dead at DN 100 and never returns.

So the formula is safe for line pipe and structural sizes and unsafe for everything a plumber handles. The sizes where it fails are ⅛ through 3½ inch, which are the sizes sold in the largest numbers — a conversion that works only above DN 100 is not much of a conversion.

Below DN 100 It Is a Lookup, Not a Formula

12 nominal sizes sit below DN 100, and 10 of them break the arithmetic. Only NPS 1 and NPS 2 land on 25 × NPS, and they do it by coincidence rather than by rule.

6, 8, 10, 15, 20, 25, 32, 40, 50, 65, 80, 90. Nothing generates that sequence from ⅛ through 3½ — it is the historic wrought-iron size series rewritten in round metric numbers, and it has to be memorised or looked up. That table is the reason a conversion page exists at all.

Read down the two columns and the pattern of the disagreement is its own kind of evidence. The DN numbers are round — 6, 8, 10, 15, 20, 25 — because they were chosen to be round, by people writing a metric series to sit alongside an existing inch one. The arithmetic column is full of quarters and eighths because that is what the inch sizes convert to. Neither column was derived from the other; they were reconciled, size by size, and where reconciliation meant rounding to a sensible metric number, that is what was done.

This is also why the pairing is stable. Because these values are conventions rather than calculations, they do not drift between editions or between standards bodies: DN 15 has meant ½ inch for as long as both designations have existed, and no revision is going to recompute it. The table is worth trusting for exactly the reason it cannot be derived.

If the inch designations themselves look wrong to you — ⅛-inch pipe measures 0.405 in across the outside — the cause is one layer further back. See nominal pipe size explained, or how to measure pipe size if you have a length of pipe in front of you.

Nominal Bore, NB and the Same Number Again

NB is numerically identical to DN. Not approximately, not by a conversion — the same integer. A supplier quoting NB 80 and a drawing calling for DN 80 are specifying the same pipe.

DN

Defined by ISO 6708. Dimensionless, written without a unit. Standard in Europe and in international specifications.

NB

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

NPS

The North American designation, in inches, used by ASME and ASTM. Above DN 100, NPS is DN divided by 25.

So DN 50, NB 50, NPS 2 and “2-inch” are four names for one pipe with an outside diameter of 60.3 mm under ASME B36.10M-2015.

Why the Same DN Is Three Different Pipes

A conversion table can match NPS 2½ to DN 65. It cannot tell you what DN 65 measures, because ISO 6708 never settled that. Its definition ties DN to the bore or the outside diameter of the end connections, and instructs each downstream standard to declare its own relationship. Most did — differently.

Same designation, three pipes, 3.3 mm apart across the widest pair. Converting NPS 2½ to DN 65 is therefore only half an answer: the other half is which standard the pipe was made to. Compiled from each standard’s own outside-diameter series — how this data was verified.

Where the Conversion Runs Out

A conversion table assumes both sides have a designation to convert. Two of the standards compiled on this site do not. GOST 8732, 8734 and 10704 name a pipe by its outside diameter and wall thickness and publish no DN column at all; GB/T 17395 is a flat list of permitted outside diameters. The DN numbers circulated for both are trade convention rather than standards text, so a chain reading “NPS 2 is DN 50 is a 57 mm Russian pipe” makes three claims of which only the first two come from a standard.

Japan and Russia each add a further name that is the same integer again. JIS writes the size with an A suffix — 50A, 65A — and that figure is the DN; Russian practice writes Ду, and in GOST 3262 the number in the designation is that value. Neither is a conversion in any real sense. They are one number in different alphabets, which is worth knowing precisely because it looks like something that needs converting.

A converted answer is not always unique either. In China both GB/T 3091 and GB/T 12459 are current standards, and they resolve DN 65 to 76.1 mm and 73.0 mm respectively — the second having been aligned to the American fitting series. A pipe and its own butt-weld fittings can therefore default to diameters 3.1 mm apart under one nominal name, inside one country, with both parties quoting a current standard. Converting the label is the easy half of the problem.

Using the Conversion With Schedules and Weights

DN and NPS name a size, never a wall thickness. A schedule number does that, and the two systems are independent: DN 50 schedule 40 and DN 50 schedule 80 share an outside diameter and differ only inside. See schedule 40 pipe dimensions for the most commonly specified series, and pipe weight per foot for what a given size and schedule weighs. For every size and schedule together, use the full pipe size chart.

Frequently Asked Questions

How do I convert NPS to DN?

At DN 100 and above, multiply the nominal pipe size by 25: NPS 12 is DN 300, NPS 24 is DN 600. Below DN 100 no multiplier works, and the twelve smallest sizes have to be looked up instead.

Is there a formula for NPS to DN?

Only above DN 100, where DN equals 25 times NPS exactly for all 32 sizes from NPS 4 to NPS 80. Below that, ten of the twelve sizes break the arithmetic. NPS 1/2 is DN 15, not DN 12.5.

Is DN 50 the same as 2 inch pipe?

Yes. DN 50, NB 50 and NPS 2 name the same pipe, and under ASME B36.10M it measures 60.3 mm across the outside. None of the three numbers is a measurement of anything on the pipe.

What is the difference between NB and DN?

Nothing numerical. NB stands for nominal bore and is the term used in India, Australia and much of the Commonwealth; DN is defined by ISO 6708 and used in Europe. NB 80 and DN 80 are the same size.

Why is DN 65 sometimes 73 mm and sometimes 76.1 mm?

Because ISO 6708 never fixed whether DN refers to the bore or the outside diameter, and left each standard to declare its own relationship. DN 65 is 73.0 mm under ASME, 76.1 mm under EN and 76.3 mm under JIS.