Nominal Bore Tables
Nominal bore, or diametre nominale, is often referred to by the abbreviation NB, or DM. Nominal bore is usually used for imperial pipe systems, and diametre nominale for metric pipe systems.
Nominal bore/diametre nominale is a value that refers to the approximate diameter of the inside of the pipe. It is useful when calculating flow rates and pressure losses; however, it cannot accurately be used to order compatible pipe and fittings. This is because the same mould can be used by manufacturers for metric and imperial fittings, and they might only change the part of the mould that changes the pipe size, and neglect to change the writing or markings in the mould.
To help avoid ordering the wrong size of fittings, check your system against the tables below.
| Pipe Size | Diametre Nominale/DM (mm) | Outside diameter of pipe (mm) |
|---|---|---|
| 16mm | 12 | 16 |
| 20mm | 16 | 20 |
| 25mm | 20 | 25 |
| 32mm | 25 | 32 |
| 40mm | 32 | 40 |
| 50mm | 40 | 50 |
| 63mm | 50 | 63 |
| 75mm | 63 | 75 |
| 90mm | 75 | 90 |
| 110mm | 90 | 110 |
| 125mm | 110 | 125 |
| 140mm | 125 | 140 |
| 160mm | 140 | 160 |
| 200mm | 160 | 200 |
| 225mm | 200 | 225 |
| 250mm | 225 | 250 |
| Pipe Size | Nominal Bore/NB | Outside diameter of pipe (mm) |
|---|---|---|
| 3/8" | 3/8" | 17.1 |
| 1/2" | 1/2" | 21.3 |
| 3/4" | 3/4" | 26.7 |
| 1" | 1" | 33.4 |
| 1 1/4" | 1 1/4" | 42.2 |
| 1 1/2" | 1 1/2" | 48.3 |
| 2" | 2" | 60.3 |
| 2 1/2" | 2 1/2" | 75 |
| 3" | 3" | 88.9 |
| 4" | 4" | 114.3 |
| 6" | 6" | 168.3 |
| 8" | 8" | 219.1 |
| 10" | 10" | 273 |
| 12" | 12" | 323.9 |
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