BSP pipe thread (G and R): tap drill chart
Full chart with the tap drill for every size. Press “Calculate” on any row to open the calculator with that thread.
BSP (British Standard Pipe, Whitworth 55° pipe thread) is the usual pipe thread in Europe. It comes in two variants: G or BSPP, parallel (ISO 228), which seals with a gasket or washer, and R/Rc or BSPT, tapered (ISO 7), which seals in the thread itself with tape or sealant. As with NPT, the nominal size is not the diameter: a G 1/4" measures 13.16 mm across the outside.
| Thread | Threads per inch | Pitch (mm) | Ø outside (mm) | Tap drill | Calculate |
|---|---|---|---|---|---|
| G 1/8-28 BSP | 28 | 0.907 | 9.728 | ⌀ 8.8 | |
| G 1/4-19 BSP | 19 | 1.337 | 13.157 | ⌀ 11.8 | |
| G 3/8-19 BSP | 19 | 1.337 | 16.662 | ⌀ 15.25 | |
| G 1/2-14 BSP | 14 | 1.814 | 20.955 | ⌀ 19 | |
| G 3/4-14 BSP | 14 | 1.814 | 26.441 | ⌀ 24.5 | |
| G 1-11 BSP | 11 | 2.309 | 33.249 | ⌀ 30.75 |
| Thread | Threads per inch | Pitch (mm) | Ø outside (mm) | Tap drill | Calculate |
|---|---|---|---|---|---|
| R 1/8-28 BSPT | 28 | 0.907 | 9.728 | ⌀ 8.4 | |
| R 1/4-19 BSPT | 19 | 1.337 | 13.157 | ⌀ 11.2 | |
| R 3/8-19 BSPT | 19 | 1.337 | 16.662 | ⌀ 14.75 | |
| R 1/2-14 BSPT | 14 | 1.814 | 20.955 | ⌀ 18.25 | |
| R 3/4-14 BSPT | 14 | 1.814 | 26.441 | ⌀ 23.75 | |
| R 1-11 BSPT | 11 | 2.309 | 33.249 | ⌀ 30 |
How to read the chart
The drill for a pipe thread does not come from subtracting the pitch: on tapered threads the diameter changes along the hole. That is why the drill from the tap makers' tables is used.
G or BSPP (parallel): the drill leaves the minor Ø within the ISO 228 tolerance; it is tapped with a straight tap and the gasket does the sealing. R/Rc or BSPT (tapered): it is tapped with a taper tap until the gauge goes in as far as the standard sets, and the thread itself does the sealing.
"Calculate" opens the calculator with that size. Guide values; on critical parts or parts under pressure, check against the tap maker's data sheet and with a gauge.
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