Pipe reference
Nominal pipe size is not the outside diameter
A plain-language NPS, IPS, CTS, and NPT reference for measuring the part that is actually available.
Reviewed against GaugeFox data and the sources below · Updated August 4, 2026
NPS / IPS outside diameters
NPS is nominal pipe size. IPS is commonly used for compatible iron-pipe-size outside diameters. The outside diameter remains fixed for a nominal size while wall thickness changes the inside diameter.
| Nominal size | Actual OD (in) | Actual OD (mm) |
|---|---|---|
| 1/8 | 0.405 | 10.29 |
| 1/4 | 0.540 | 13.72 |
| 3/8 | 0.675 | 17.15 |
| 1/2 | 0.840 | 21.34 |
| 3/4 | 1.050 | 26.67 |
| 1 | 1.315 | 33.40 |
| 1 1/4 | 1.660 | 42.16 |
| 1 1/2 | 1.900 | 48.26 |
| 2 | 2.375 | 60.33 |
CTS outside diameters
Copper tube size is another nominal family used by copper and many PEX systems. Its outside diameters are different from NPS / IPS, even when the trade-size label is the same.
| Nominal size | Actual OD (in) | Actual OD (mm) |
|---|---|---|
| 1/4 | 0.375 | 9.53 |
| 3/8 | 0.500 | 12.70 |
| 1/2 | 0.625 | 15.88 |
| 5/8 | 0.750 | 19.05 |
| 3/4 | 0.875 | 22.23 |
| 1 | 1.125 | 28.58 |
| 1 1/4 | 1.375 | 34.93 |
| 1 1/2 | 1.625 | 41.28 |
| 2 | 2.125 | 53.98 |
Choose the surface you can trust
- On straight smooth pipe or an unthreaded nipple section, measure the body OD.
- For a larger installed smooth section, measure around it and divide by π.
- On a fully threaded male NPT nipple, compare the threaded end with an NPT identification reference.
- For a female fitting, use attached smooth pipe, a mating male part, or the cast or printed marking.
What OD cannot tell you
Outside diameter can identify a likely trade-size family. It cannot prove wall thickness, schedule, pressure rating, material, thread conformance, or whether the part is suitable for an installation.
Reference trail
Sources and review method
These source pages identify the standards or industry references used for this guide. Full standards may require purchase. The guide is an identification aid, not a substitute for a calibrated inspection or the complete applicable standard.
Read the editorial standardUse it at the bench
Put these references on your Android screen.
GaugeFox adds calibrated on-screen gauges, searchable nearby sizes, and a local Parts Bin to the reference data in this guide.