Pipe reference
How to size pipe by measuring around it
A reliable circumference method for pipe that is installed, larger than a caliper, or too wide for the phone screen.
Reviewed against GaugeFox data and the sources below · Updated August 4, 2026
Make one level loop
- Choose a clean, smooth section with no fitting, seam bulge, or thread.
- Wrap a flexible tape or nonstretch string once around the pipe.
- Keep the loop square to the pipe—not spiraled—and snug without crushing soft tube.
- Mark the exact overlap, then measure the loop length.
- Divide that length by 3.1416 and compare the result with a pipe OD table.
Worked example
Suppose the distance around the pipe is 67.0 mm. Divide 67.0 by 3.1416 to get 21.33 mm. That is essentially the 21.34 mm standard OD of 1/2 inch NPS / IPS pipe.
It is not 1/2 inch CTS: that family has a 15.88 mm OD. The family still matters after the math.
Useful NPS circumference checks
| Nominal NPS | Body OD | Distance around |
|---|---|---|
| 1/2 | 21.34 mm | 67.04 mm |
| 3/4 | 26.67 mm | 83.79 mm |
| 1 | 33.40 mm | 104.94 mm |
| 1 1/4 | 42.16 mm | 132.45 mm |
| 1 1/2 | 48.26 mm | 151.61 mm |
| 2 | 60.33 mm | 189.53 mm |
When the formula gives a believable wrong answer
- A threaded section measures across thread crests, not the smooth pipe body.
- A tapered fitting changes diameter along its length.
- Pipe-jaw marks, heavy paint, insulation, and corrosion add circumference.
- A hex-shaped or ribbed fitting is not circular, so C ÷ π does not describe its useful size.
- Flexible tube that is flattened or oval needs measurements in more than one direction.
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.