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OD / ID / wall calculator
Three numbers describe a pipe round: outer diameter, inner diameter, wall thickness. Only two are independent — give any two and the third follows from ID = OD − 2 × wall.
The triangle, in both directions
| Known | Compute | Watch out for |
|---|---|---|
| OD + wall | ID = OD − 2 × wall | If the wall is a minimum (PVC, PEX), the ID you get is a maximum. |
| OD + ID | wall = (OD − ID) / 2 | A worn or out-of-round pipe gives a wall that is not the nominal wall. |
| ID + wall | OD = ID + 2 × wall | Bore measured with a caliper leg is usually not a true diameter. |
Worked from published values
| Size | OD (in) | OD (mm) | Wall (in) | ID (in) | ID (mm) | ≈ kg/m |
|---|---|---|---|---|---|---|
| NPS 1/2 40 | 0.840 | 21.336 | 0.109 | 0.622 | 15.799 | 1.27 |
| NPS 2 80 | 2.375 | 60.325 | 0.218 | 1.939 | 49.251 | 7.48 |
| PVC 1 40 | 1.315 | 33.401 | 0.133 | 1.049 | 26.645 | 0.45 |
Mass uses ρ = 7850 kg/m³ for steel and 1400 kg/m³ for PVC: it is a derived estimate, not a published weight table. Wall values in plastics are minimums.
What the third number does not tell you
- Pressure rating — depends on material and temperature, not just wall.
- Which fitting fits — the OD decides that, so it is the number to measure: reverse lookup.
- The nominal name — steel, PVC, PEX/CTS and stainless charts list what each name is worth.