Pipe sizes are one of the most confusing places to convert between inches and millimeters, because the “size” printed on a pipe is usually not its actual measurement. A “1/2-inch” steel pipe is not 1/2 inch across on the outside or the inside. This guide explains nominal sizes, gives the real outside diameters in inches and mm, and shows how to match inch and metric pipe.
Nominal Size vs Actual Size
In the US, steel and PVC pipe is sold by Nominal Pipe Size (NPS). In metric countries the equivalent label is DN (diamètre nominal), a dimensionless number that only loosely corresponds to millimeters. Both are names, not measurements:
- The outside diameter (OD) for each nominal size is fixed, so fittings from different makers fit together.
- The inside diameter changes with wall thickness (the “schedule” — Schedule 40, Schedule 80 and so on). A thicker wall means a smaller inside.
That is why you should always measure the outside of a pipe to identify it.
Steel and PVC Pipe Sizes (NPS) in Inches and MM
| Nominal size (NPS) | Metric name | Outside diameter (in) | Outside diameter (mm, rounded to 0.1) |
|---|---|---|---|
| 1/8″ | DN 6 | 0.405″ | 10.3 mm |
| 1/4″ | DN 8 | 0.540″ | 13.7 mm |
| 3/8″ | DN 10 | 0.675″ | 17.1 mm |
| 1/2″ | DN 15 | 0.840″ | 21.3 mm |
| 3/4″ | DN 20 | 1.050″ | 26.7 mm |
| 1″ | DN 25 | 1.315″ | 33.4 mm |
| 1-1/4″ | DN 32 | 1.660″ | 42.2 mm |
| 1-1/2″ | DN 40 | 1.900″ | 48.3 mm |
| 2″ | DN 50 | 2.375″ | 60.3 mm |
Notice that for small sizes the outside diameter is much bigger than the name suggests: 1/2″ pipe is actually 0.840″ (21.3 mm) across the outside.
Copper Tube: A Different Rule
US copper water tube (Types K, L and M) is also named by a nominal size, but the rule is different: the outside diameter is always 1/8 inch larger than the nominal size. Metric copper, used in the UK and Europe, is named by its actual outside diameter.
| US copper nominal | Actual OD (in) | Actual OD (mm, rounded to 0.01) | Closest metric copper |
|---|---|---|---|
| 1/2″ | 0.625″ | 15.88 mm | 15 mm |
| 3/4″ | 0.875″ | 22.23 mm | 22 mm |
| 1″ | 1.125″ | 28.58 mm | 28 mm |
US 3/4″ copper (22.23 mm OD) and metric 22 mm copper differ by only about 0.2 mm (22.23 mm vs 22.00 mm), but use a fitting only if its label lists both sizes. Compression fittings and solder joints need the exact size.
Worked Example: Identify a Pipe from Its Circumference
Suppose a paper strip wrapped once around a clean pipe measures 84 mm. Divide by π: 84 ÷ 3.1416 ≈ 26.74 mm outside diameter. That is close to the 26.7 mm entry for 3/4-inch NPS pipe in the table, rather than an actual 3/4-inch diameter of 19.05 mm.
Use this as an identification clue. Paint, insulation, an uneven wrap and the strip’s thickness affect the result. Measure a clean section with calipers when possible, then check the pipe marking, material, wall thickness and connection type before selecting a fitting. Outside diameter alone does not establish compatibility.
For US copper tube, consult the Copper Development Association’s Copper Tube Handbook: Types K, L and M use an outside diameter 1/8 inch larger than their standard size designation.
How to Identify an Unknown Pipe
- Measure the outside diameter with calipers, or wrap a strip of paper around the pipe, mark the overlap, and divide that circumference by π (about 3.1416).
- Convert to both inches and mm, then compare with the tables above.
- Check the material: steel and PVC follow NPS, US copper follows the “+1/8 inch” rule, metric copper and metric plastic pipe follow the actual OD.
Example: a pipe with a 26.7 mm outside diameter is 1.05″ — that is 3/4″ NPS (DN 20), not 1 inch.
Pipe Threads
Pipe threads also differ between systems. North America uses NPT (tapered); much of the rest of the world uses BSP (BSPT tapered or BSPP parallel). A 1/2″ NPT and a 1/2″ BSP thread are close in size but have different thread angles (60° for NPT, 55° for BSP) and, in some sizes, different pitches, so they will not seal properly together. Use an adapter rather than forcing them.
Frequently Asked Questions
What is 1/2 inch pipe in mm?
1/2″ NPS pipe has an outside diameter of 0.840″, which is 21.3 mm. Its metric name is DN 15.
Is 22 mm pipe the same as 3/4 inch?
For copper, 22 mm is very close to US 3/4″ copper (22.23 mm OD). For steel pipe, 3/4″ NPS is 26.7 mm OD, so it is not the same.
Why is a 1 inch pipe bigger than 1 inch?
Because “1 inch” is a nominal name. The outside diameter of 1″ NPS pipe is 1.315″ (33.4 mm).
Related: Metric bolt sizes in inches · Metric vs imperial explained · Large sizes in inches
Sources and Checking
- NIST Guide to the SI, Appendix B.8 (1 inch = 0.0254 m exactly)
- ASME B36.10M, Welded and Seamless Wrought Steel Pipe (outside diameters by NPS)
Published by Johnson, who runs mmintoinches.com. Every value in this guide was recalculated from 1 inch = 25.4 mm and checked against the sources below on 9 October 2026. Found a mistake? Email support@mmintoinches.com with the page and the value.