Pipe & tube reference · ASME · ASTM
Outer diameter, wall thickness and bore — with the sources beside the number
A nominal size names a pipe; it does not measure one. A “1/2 inch” pipe is0.840 in / 21.336 mm on the outside as steel, and a completely different 0.625 in / 15.875 mm as PEX tubing. This site publishes the measured dimension for each standard, and says which two sources agree on it.
I measured a pipe — what is it?
The result page lists every standard that fits your measurement within the tolerance you choose, and tells you which measurement separates them.
The five sizes people look up most
ASME B36.10M welded and seamless steel pipe, Schedule 40. The OD is fixed by size; the schedule only changes the wall.
| Nominal (NPS) | DN | OD (in) | OD (mm) | Sch 40 wall (in) | Sch 40 ID (in) |
|---|---|---|---|---|---|
| NPS 1/2 | 15 | 0.840 | 21.336 | 0.109 | 0.622 |
| NPS 3/4 | 20 | 1.050 | 26.67 | 0.113 | 0.824 |
| NPS 1 | 25 | 1.315 | 33.401 | 0.133 | 1.049 |
| NPS 2 | 50 | 2.375 | 60.325 | 0.154 | 2.067 |
| NPS 4 | 100 | 4.500 | 114.3 | 0.237 | 4.026 |
Charts by standard
ASME B36.10M
Welded and seamless wrought steel pipe. OD is fixed per size; the wall (schedule) is what you choose.
17 sizes · 128 published values · 17 awaiting a second source
ASTM D2241
PVC pressure pipe (CTS-adjacent iron pipe sizes). Walls are minimums, so a derived bore is a maximum.
9 sizes · 18 published values
ASTM F876/F877 (CTS)
Crosslinked polyethylene tubing in CTS (copper tube size) ODs, designated by SDR rather than schedule.
9 sizes · 3 published values · 6 awaiting a second source
ASME B36.19M
Stainless wrought pipe. Shares the B36.10 ODs; walls carry an S suffix (5S, 10S, 40S, 80S).
17 sizes · 64 published values · 4 awaiting a second source
Tools
- OD / ID / wall calculator — give two, get the third, plus flow area, volume per metre and linear weight.
- Reverse lookup — measured OD (and optionally wall) to the standards that fit.
- Inch ↔ millimetre for pipe ODs — the conversions that actually appear on pipe, not generic unit maths.
- NPS vs DN, NPS vs PVC, NPS vs PEX/CTS — same label, different body.
Four words that cause most of the confusion
- Outer diameter (OD)
- What calipers measure and what a clamp, a hole saw or a insulation sleeve has to fit. It is fixed for a given nominal size, whatever the wall.
- Inner diameter (ID, bore)
- OD − 2 × wall. It is the number that decides flow, and it changes with the schedule: NPS 2 Sch 40 bores 2.067 in, Sch 80 bores 1.939 in.
- Wall thickness
- The part you choose. Steel uses schedules (10…160) and the definitions STD / XS / XXS; plastics use pressure ratings; PEX uses SDR.
- Nominal (NPS / DN)
- A name. NPS 1/2 is not 0.5 in wide — its OD is 0.840 in. DN is the millimetre-family label (DN 15 ≈ NPS 1/2) and is also not the OD.
Before you cut, weld or order: these are the published standard dimensions, not a substitute for the specification your project is built to. Where a source publishes a minimum wall (PVC, PEX), the bore we derive is a maximum — the pipe in your hand can be slightly thicker and the bore slightly smaller.How values are verified.