A nominal size is not measured
DN, NPS and the JIS A size are names, not diameters: a DN15 or NPS 1/2 pipe has a 21.3 mm outside diameter. A caliper checks the outside diameter, never the nominal size.
Reference tool — not inspection
Identify the pipe before it goes in an RFQ.
Pipe
A measurement or a nominal size.
The outside diameter or the circumference is enough; add the wall thickness for the ASME schedule. Below the examples, an offered size (DN65, NPS 2 1/2, 65A) can be checked against the fitted pipe on the outside diameter. The result updates as you type.
Examples:
Replacement check
On the outside diameters alone: the wall, the material and the pressure rating decide.
All 130 engineering tools
Couplings, clamps, slip-on flanges and pipe spools are ordered by the pipe they fit, and the nominal size is not the outside diameter. DN65 is 76.1 mm in the ISO/EN series, 73.0 mm in ASME B36.10 (NPS 2 1/2) and 76.3 mm in JIS G3452 (65A). Measure the outside diameter or the circumference, and the tool lists the sizes that fit.
Some series sit closer together than a caliper can resolve — DN65 and 65A are 0.2 mm apart — and the tool says so instead of picking one for you. The pipe marking, the flange or the drawing settles it.
Why pipes get confused
Nominal size, three series, the fit.
Three series, small differences
For DN25 the ISO/EN series gives 33.7 mm, ASME 33.4 and JIS 34.0. A difference of a few tenths is often less than a reading over paint can resolve, so the tool shows every close size.
Where the difference decides
Couplings, clamps, supports and slip-on flanges are made to the outside diameter. At DN65 the ISO/EN and ASME series are more than 3 mm apart, and a part for one will not fit the other.
| Size | ISO / EN, mm | ASME B36.10, mm | JIS G3452, mm |
|---|---|---|---|
| DN15 · 1/2 · 15A | 21.3 | 21.3 | 21.7 |
| DN20 · 3/4 · 20A | 26.9 | 26.7 | 27.2 |
| DN25 · 1 · 25A | 33.7 | 33.4 | 34.0 |
| DN32 · 1 1/4 · 32A | 42.4 | 42.2 | 42.7 |
| DN40 · 1 1/2 · 40A | 48.3 | 48.3 | 48.6 |
| DN50 · 2 · 50A | 60.3 | 60.3 | 60.5 |
| DN65 · 2 1/2 · 65A | 76.1 | 73.0 | 76.3 |
| DN80 · 3 · 80A | 88.9 | 88.9 | 89.1 |
| DN90 · 3 1/2 | — | 101.6 | — |
| DN100 · 4 · 100A | 114.3 | 114.3 | 114.3 |
| DN125 · 5 · 125A | 139.7 | 141.3 | 139.8 |
| DN150 · 6 · 150A | 168.3 | 168.3 | 165.2 |
Highlighted rows have series more than 1 mm apart.
Copper-nickel pipe
Copper-nickel 90/10 pipe to EEMUA 144 and DIN 86019 has its own outside diameters; the tool knows 16, 25, 30, 38, 44.5, 57, 76.1, 88.9, 108 and 159 mm. At 76.1 and 88.9 mm it shares the diameter of steel pipe, so the material has to be in the request.
Sources
ISO / EN: EN 10220 outside diameters (Project Materials’ table), the same set as ArcelorMittal’s EN 10217 and EN 10255 range; DN pairing as in the EN 10255 size table. ASME B36.10: InfraBuild’s table (DN, NPS, outside diameter). JIS G3452 (SGP): Nippon Steel’s delivery drawings for SGP pipe, 15A to 150A, which allow 0.5 mm either way on the outside diameter of the small sizes. Sizes above DN150 are left out. Copper-nickel: World Wide Metric’s copper-nickel metric pipe sheet and piping-world’s EEMUA 144 table; only the diameters both give are used. Wall schedules: the Wikipedia NPS tables (inch walls converted to mm) and Engineering ToolBox’s ASME B36.10 / B36.19 table, which agree to within 0.03 mm; the wall may run up to 12.5 % under nominal under ASTM A53 and A106.
Before you send the pipe
Questions about pipes in an RFQ
Is DN65 the same as 2 1/2 inch?
The same nominal size, not the same pipe: 76.1 mm outside diameter in the ISO/EN series, 73.0 mm in ASME B36.10 and 76.3 mm in JIS G3452 (65A). Couplings, clamps and slip-on flanges follow the outside diameter, so the request should give the measured diameter as well as the size.
Why does a JIS 25A pipe measure 34.0 mm?
JIS G3452 has its own outside diameters: 34.0 mm for 25A, where the ISO/EN series has 33.7 mm and ASME 33.4 mm for the same nominal size.
What does Sch 40 or Sch 80 tell me?
The wall of an ASME pipe. Every schedule of one NPS size has the same outside diameter and a different wall, so the bore changes: NPS 2 has a 3.91 mm wall in Sch 40 and 5.54 mm in Sch 80. Up to NPS 6, Sch 40 is the STD wall and Sch 80 the XS wall. Enter the wall thickness with the diameter and the tool names the schedule it reads as.
How do I measure a large pipe?
Wrap a tape round it and enter the circumference; the outside diameter is the circumference divided by 3.1416. Measure on bare metal, away from welds, insulation and thick paint.
What if nothing matches?
Check the reading in two directions. Plastic pipe, hydraulic tube, steel pipe above DN150 and copper-nickel sizes outside the table are not covered; send the measured diameter, the wall thickness if a cut end shows it, and a photograph of any marking.
Pipe identified?
Send the measurement with the photograph.
An identified size is a cross-check on your measurement, not proof. Give the measured diameter, the wall thickness, the material and any marking, and Vessel Core checks them against each other before an offer goes back.
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