DN25 is one inch. DN50 is two inches. DN100 is four inches. Almost every engineer knows those three by heart, and almost every DN to inch table online stops there. The trouble starts at the edges: DN20 is not a 20 mm pipe, DN32 is not 1.26 inches in any arithmetic sense, and DN1400 has no inch value you can guess.
This page gives the full nominal table from DN6 to DN3000 and, more usefully than the table itself, the rule that generates it, so any size missing from a drawing legend can still be derived in ten seconds.
The rule that covers most sizes: DN = 25 x NPS
For NPS 4 and larger, DN = 25 multiplied by NPS. ASME B16.5 states it outright: for NPS of 4 and above, the related DN is 25 times the NPS. That single line is why NPS 4 is DN100, NPS 8 is DN200, NPS 24 is DN600 and NPS 40 is DN1000.
Two things follow from it. First, a DN value is always a round number - 100, 150, 600 - never 101.6, because it is a designation and not a measurement. Second, the multiplier is 25, not 25.4. DN is a metric size label, not a unit conversion, and the 1.6 percent you would gain from 25.4 does not exist in any standard.
The cost of confusing the two is easy to state: DN600 is NPS 24, yet the pipe arrives with an outside diameter of 609.6 mm. DN100 is NPS 4, and the outside diameter is 114.3 mm. If a fabricator reads DN as millimetres and cuts a 600 mm opening, the 609.6 mm pipe does not fit.
Where the rule breaks: DN6 to DN90
Below NPS 4 the multiplier fails, because those sizes were fixed long before the metric designation existed.
- NPS 1/2 is DN15, not DN12.5.
- NPS 3/4 is DN20, not DN19.
- NPS 1 is DN25. NPS 1-1/4 is DN32, not 31.75.
- NPS 1-1/2 is DN40, NPS 2 is DN50, NPS 2-1/2 is DN65, NPS 3 is DN80, NPS 3-1/2 is DN90.
Not one of them can be calculated; the block has to be memorized or looked up.
Below DN15 the series turns fractional and belongs mostly to instrument tubing, small-bore fittings and sampling lines: DN6 is 1/8, DN8 is 1/4, DN10 is 3/8. A yard that stocks DN15 and up will still order these on demand rather than hold them.
DN25 to inch, and why that answer is only half right
DN25 to inch is the most searched conversion on this subject, and the answer - 1 inch - is correct as a size designation and wrong as a dimension.
A DN25 steel pipe has an outside diameter of 33.4 mm, which is 11.6 mm more than the 25.4 mm that one inch actually measures. The "1 inch" refers to the nominal bore of the old iron pipe standard the designation grew out of, not to the pipe in the truck.
The same shift appears at every small size. DN50 to inch gives 2, but the outside diameter is 60.3 mm rather than 50.8. DN100 gives 4, with an outside diameter of 114.3 mm. DN32 gives 1-1/4, with 42.2 mm. The inch number and the millimetre number describe two different things, and a drawing that shows only one of them is ambiguous.
The safe convention is to write both: DN25 (NPS 1), outside diameter 33.4 mm. Once the outside diameter is on the drawing, no one can read the size two ways.
The complete chart, DN6 to DN1200
Outside diameters below are the ASME B36.10M carbon steel and B36.19M stainless steel series values. From NPS 1/8 through NPS 12 the outside diameter is larger than the inch number; from NPS 14 upward the outside diameter equals the inch number times 25.4, which is why it matches the NPS column exactly.
| DN | NPS (inch) | Outside diameter (mm) | Outside diameter (inch) |
| DN6 | 1/8 | 10.3 | 0.405 |
| DN8 | 1/4 | 13.7 | 0.540 |
| DN10 | 3/8 | 17.1 | 0.675 |
| DN15 | 1/2 | 21.3 | 0.840 |
| DN20 | 3/4 | 26.7 | 1.050 |
| DN25 | 1 | 33.4 | 1.315 |
| DN32 | 1-1/4 | 42.2 | 1.660 |
| DN40 | 1-1/2 | 48.3 | 1.900 |
| DN50 | 2 | 60.3 | 2.375 |
| DN65 | 2-1/2 | 73.0 | 2.875 |
| DN80 | 3 | 88.9 | 3.500 |
| DN90 | 3-1/2 | 101.6 | 4.000 |
| DN100 | 4 | 114.3 | 4.500 |
| DN125 | 5 | 141.3 | 5.563 |
| DN150 | 6 | 168.3 | 6.625 |
| DN200 | 8 | 219.1 | 8.625 |
| DN250 | 10 | 273.0 | 10.750 |
| DN300 | 12 | 323.9 | 12.750 |
| DN350 | 14 | 355.6 | 14.000 |
| DN400 | 16 | 406.4 | 16.000 |
| DN450 | 18 | 457.2 | 18.000 |
| DN500 | 20 | 508.0 | 20.000 |
| DN550 | 22 | 558.8 | 22.000 |
| DN600 | 24 | 609.6 | 24.000 |
| DN650 | 26 | 660.4 | 26.000 |
| DN700 | 28 | 711.2 | 28.000 |
| DN750 | 30 | 762.0 | 30.000 |
| DN800 | 32 | 812.8 | 32.000 |
| DN850 | 34 | 863.6 | 34.000 |
| DN900 | 36 | 914.4 | 36.000 |
| DN950 | 38 | 965.2 | 38.000 |
| DN1000 | 40 | 1016.0 | 40.000 |
| DN1050 | 42 | 1066.8 | 42.000 |
| DN1100 | 44 | 1117.6 | 44.000 |
| DN1150 | 46 | 1168.4 | 46.000 |
| DN1200 | 48 | 1219.2 | 48.000 |
Large sizes, DN1300 to DN3000
Above DN1200 the DN series thins out. ASME B16.47 stops at NPS 60 in most classes, and above that pipe is normally bought by outside diameter and wall thickness rather than by nominal size. The rule still holds, so intermediate sizes can be filled in: DN1450 is NPS 58, DN1750 is NPS 70, DN2100 is NPS 84. Only the rows below are common enough to keep on a desk.
| DN | NPS (inch) | Outside diameter (mm) |
| DN1300 | 52 | 1320.8 |
| DN1400 | 56 | 1422.4 |
| DN1500 | 60 | 1524.0 |
| DN1600 | 64 | 1625.6 |
| DN1700 | 68 | 1727.2 |
| DN1800 | 72 | 1828.8 |
| DN1900 | 76 | 1930.4 |
| DN2000 | 80 | 2032.0 |
| DN2200 | 88 | 2235.2 |
| DN2400 | 96 | 2438.4 |
| DN2600 | 104 | 2641.6 |
| DN2800 | 112 | 2844.8 |
| DN3000 | 120 | 3048.0 |
What the chart does not tell you
A nominal size chart answers one question - which size family this part belongs to. It answers none of the questions that decide whether the part works. Three of them matter more than the conversion itself.
Schedule moves the bore, never the size
A DN100 pipe is DN100 whether it is Schedule 10, Schedule 40 or Schedule 80. The outside diameter stays at 114.3 mm in all three. Only the wall moves, and with it the bore: Schedule 40 wall is 6.02 mm and leaves an inside diameter of about 102.3 mm, while Schedule 80 wall is 8.56 mm and leaves about 97.2 mm.
Five millimetres of bore is the whole flow-area question in a revamp, and it is invisible in every DN to inch chart ever printed.
The practical consequence: never size a pump, a control valve or a relief line from DN. Use the inside diameter for the schedule you are actually buying, computed as outside diameter minus twice the wall thickness. DN is a label; the bore is the physics.
A matching DN does not mean the parts bolt together
DN100 and NPS 4 identify the same nominal size. A DN100 PN16 flange and an NPS 4 Class 150 flange are not automatically interchangeable, because their bolt circle, bolt count, bolt diameter, flange thickness and facing can all differ. DN tells you the size family. The dimensional standard - B16.5, B16.47, EN 1092-1 - tells you whether the two will actually mate.
The same caution applies across product families. A DN50 valve bore, a DN50 pipe and a DN50 flange are the same nominal size and three separate dimension sets. Buying all three on the DN number alone is how mismatched gaskets and refaced flanges end up on a purchase order.
Three mistakes this table exists to prevent
- Reading DN as millimetres. DN25 is not a 25 mm pipe; it is a 33.4 mm outside diameter. A 25 mm hole will not accept it.
- Converting by dividing by 25.4. DN25 divided by 25.4 gives 0.98, which happens to round to 1 inch and teaches the wrong method. DN32 divided by 25.4 gives 1.26, which only reads as 1-1/4 if you already knew the answer.
- Ordering "2 inch pipe" and expecting 50.8 mm. NPS 2 arrives at 60.3 mm outside diameter, and a clamp, a saddle or a penetration cut for 50.8 mm will be undersized.
How to identify an unmarked pipe in the yard
Identification runs on the outside diameter, because that is the one dimension that does not change with schedule, corrosion allowance or lining. The sequence takes three measurements.
- Caliper the outside diameter. 33.4 mm is NPS 1 (DN25), 60.3 mm is NPS 2 (DN50), 114.3 mm is NPS 4 (DN100), 168.3 mm is NPS 6 (DN150), 219.1 mm is NPS 8 (DN200). Those five cover most of what sits on a rack.
- Measure the wall with an ultrasonic gauge, or read the stencil if the pipe still carries one. Compare the wall against the schedule table for that size - the same schedule number is a different thickness at every size.
- Confirm the material grade from the mill certificate or the stamp before the pipe goes into a pressure line. A correct DN with an unverified grade is still an unverified component.
Inside diameter cannot be used for identification. It shifts with wall thickness, with corrosion, and with any lining or coating applied after the pipe was made.
Quick reference for the sizes that carry most orders
If a drawing set is being checked rather than written, these ten sizes are almost always the ones in dispute. The outside diameters are the values to compare against a physical measurement.
| DN | NPS (inch) | Outside diameter (mm) | Where it usually appears |
| DN15 | 1/2 | 21.3 | Instrument air, small-bore chemical feed, sampling |
| DN25 | 1 | 33.4 | Utility headers, small pumps, CIP lines |
| DN50 | 2 | 60.3 | Branch lines, small exchangers, general process |
| DN80 | 3 | 88.9 | Pump discharge, small headers |
| DN100 | 4 | 114.3 | Main process lines on skids, common flange size |
| DN150 | 6 | 168.3 | Headers, larger exchangers, tank nozzles |
| DN200 | 8 | 219.1 | Main headers, seawater and cooling lines |
| DN300 | 12 | 323.9 | Trunk lines, large pumps, storage transfer |
| DN400 | 16 | 406.4 | Large trunk lines, terminal piping |
| DN600 | 24 | 609.6 | Sea water intakes, large storage and loading lines |
One closing point on how these numbers travel. A DN value on its own is a size family; a purchase order that satisfies a mill certificate needs the outside diameter, the wall thickness, the material grade and the dimensional standard as well. Charts like this one exist to get the first number right quickly, not to replace the other four.
Want the reasoning behind the numbers? The step-by-step method, the worked examples, and the DN versus NPS versus outside diameter discussion are in How to Convert DN to Inches.
