BSP Tap Drill Size Calculator
Tapping drill sizes for 1/8″ to 2″ BSP pipe threads — threading steel, cast iron and aluminium
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Select BSP Size
BSP Pipe Size
Tapping Drill
18.60mm
OD
20.96mm
TPI
14
Important: BSP nominal sizes do not correspond to measured dimensions. Always use the reference table — never calculate from the size name.
Full BSP Tapping Drill Reference Table
Click any row to select that size above. All dimensions in millimetres.
| BSP Size | OD (mm) | TPI | Tapping Drill (mm) |
|---|---|---|---|
| 1/8" BSP | 9.73 mm | 28 | 8.80 mm |
| 1/4" BSP | 13.16 mm | 19 | 11.80 mm |
| 3/8" BSP | 16.66 mm | 19 | 15.00 mm |
| 1/2" BSP | 20.96 mm | 14 | 18.60 mm |
| 5/8" BSP | 22.91 mm | 14 | 20.60 mm |
| 3/4" BSP | 26.44 mm | 14 | 24.30 mm |
| 7/8" BSP | 30.20 mm | 14 | 27.80 mm |
| 1" BSP | 33.25 mm | 11 | 30.50 mm |
| 1-1/4" BSP | 41.91 mm | 11 | 39.20 mm |
| 1-1/2" BSP | 47.80 mm | 11 | 45.10 mm |
| 2" BSP | 59.61 mm | 11 | 57.20 mm |
All values are for standard BSP (British Standard Pipe) parallel threads. Tapping drill sizes are for approximately 75% thread engagement.
BSPP vs BSPT — Which Thread Do You Have?
BSPP — Parallel (G Thread)
The thread diameter is constant along its length. The most common form on UK and European farm machinery, hydraulic fittings and plumbing. The data in this calculator applies to BSPP. Sealing is achieved with a bonded seal, O-ring or PTFE tape.
BSPT — Tapered (R Thread)
The thread tapers at 1 in 16 (3.58°) so the thread itself creates the seal when tightened. Less common on modern fittings but found on older cast iron bodies and some gas fittings. BSPT and BSPP share the same TPI and pitch — tapping drill sizes are identical — but the threads are not interchangeable under pressure without a suitable sealant.