Conversion · pipe sizes only
Pipe outer diameter: inches ↔ millimetres
The inch value is the definition; the millimetre value is that number times 25.4. Since 1959 the inch is exactly 25.4 mm, so nothing here is rounded from a chart — read the inch column to quote a standard, the millimetre column to match a measurement.
Every published OD, both units
| OD (in) | OD (mm) | Nominal | Standards |
|---|---|---|---|
| 0.405 | 10.287 | 1/8 | steel · stainless |
| 0.540 | 13.716 | 1/4 | steel · stainless |
| 0.625 | 15.875 | 1/2 | PEX |
| 0.675 | 17.145 | 3/8 | steel · stainless |
| 0.840 | 21.336 | 1/2 | steel · PVC · stainless |
| 0.875 | 22.225 | 3/4 | PEX |
| 1.050 | 26.670 | 3/4 | steel · PVC · stainless |
| 1.125 | 28.575 | 1 | PEX |
| 1.315 | 33.401 | 1 | steel · PVC · stainless |
| 1.660 | 42.164 | 1 1/4 | steel · stainless |
| 1.900 | 48.260 | 1 1/2 | steel · PVC · stainless |
| 2.375 | 60.325 | 2 | steel · PVC · stainless |
| 2.875 | 73.025 | 2 1/2 | steel · stainless |
| 3.500 | 88.900 | 3 | steel · PVC · stainless |
| 4.500 | 114.300 | 4 | steel · PVC · stainless |
| 5.563 | 141.300 | 5 | steel · stainless |
| 6.625 | 168.275 | 6 | steel · PVC · stainless |
| 8.625 | 219.075 | 8 | steel · PVC · stainless |
| 10.750 | 273.050 | 10 | steel · stainless |
| 12.750 | 323.850 | 12 | steel · stainless |
Why the millimetre column matters more than it looks
Two sizes can sit a fraction of a millimetre apart and belong to completely different pipes: a copper tube (not published here) and a steel tube can both read “about 22 mm”. The chart below your caliper reading is the one that names it — reverse lookup does the same job with a tolerance you choose.
DN is not a millimetre measurement of the pipe either: it is a name from the metric family. NPS ↔ DN shows where the two naming systems meet and where one has no twin.