G1/2 BSP Thread Dimensions and Tap Drill
G1/2 is the parallel British Standard Pipe thread (BSPP) of nominal size 1/2: 14 threads per inch on a 55° Whitworth form with rounded crests and roots, and a major diameter of 20.955 mm (0.8250 in). The size names the bore of the pipe it was once cut on, so nothing on the thread measures 1/2 inch.
G1/2 is the standard thread for taps, shower hoses and radiator valves in Europe.
- Tap drill19 mm0.7480 in, 84% thread
- Threads per inch14pitch 1.814 mm
- Major diameter20.955 mm0.8250 in
- Pitch diameter19.793 mm0.7793 in
- Minor diameter18.632 mm0.7335 in
- Thread height1.162 mm0.0457 in
Thread profile
G1/2 dimensions
| Dimension | mm | in |
|---|---|---|
| Major diameter d = D | 20.955 | 0.8250 |
| Pitch diameter d2 = D2 | 19.793 | 0.7793 |
| Minor diameter d1 = D1 | 18.632 | 0.7335 |
| Pitch P | 1.814 | 0.07143 |
| Thread height h (0.640327 × P) | 1.162 | 0.0457 |
| Crest and root radius r (0.137329 × P) | 0.249 | 0.0098 |
| Height of fundamental triangle H | 1.743 | 0.0686 |
| Property | Value |
|---|---|
| Threads per inch | 14 |
| Flank angle | 55° (27.5° each side) |
| Taper | none (parallel thread) |
Basic dimensions from ISO 228-1 Table 1. Inch values are conversions.
G1/2 tap drill size
Tap drill for G1/2: 19 mm (0.7480 in)
This is 0.37 mm over the basic minor diameter of 18.632 mm and leaves about 84% thread.
ISO 228-1 puts the whole tolerance on the minor diameter of the internal thread above the basic size, so the hole is always drilled larger than 18.632 mm. Drilling to the basic minor diameter itself leaves the tap cutting on its full profile and risks breaking it.
| Drill | mm | in | % thread | Thread engagement |
|---|---|---|---|---|
| 18.75 mm | 18.75 | 0.7382 | 95% | |
| 19 mmrecommended | 19.00 | 0.7480 | 84% | |
| 19.25 mm | 19.25 | 0.7579 | 73% | |
| 19.5 mm | 19.50 | 0.7677 | 63% | |
| 19.75 mm | 19.75 | 0.7776 | 52% |
% thread = (major diameter − drill diameter) ÷ (2 × thread height).
G1/2 and the tapered R1/2 (BSPT)
The taper thread of ISO 7-1 (EN 10226), written R1/2 for the male and Rc1/2 or Rp1/2 for the female, has the same 14 threads per inch and the same 55° form as G1/2, and at its gauge plane it has the same diameters. It differs in being cut on a 1 in 16 taper and in sealing on the thread with a sealant. The male R1/2 mates with a taper Rc1/2 or a parallel Rp1/2 female thread. Rp is not the same thing as G: the basic sizes agree but the tolerances do not, so a G1/2 port is not a specified mate for a taper male. The tap drill for the taper internal thread Rc1/2 is 17.8 mm, and for the parallel internal thread Rp1/2 it is 18.5 mm; both are smaller than the 19 mm used for G1/2.
How G1/2 seals
A G1/2 joint does not seal on its threads. ISO 228-1 is the standard for pipe threads where the pressure-tight joint is made outside the thread: the male fitting clamps a bonded washer, an O-ring or a flat gasket against the face of the port. The thread only supplies the clamping force, so thread sealant is not what makes the joint tight.
G1/2 against 1/2 NPT
The American NPT thread of the same nominal size has 14 threads per inch against 14, a 60° flank angle and an outside diameter of 21.336 mm against 20.955 mm. The pitch is the same, so the parts start to screw together, but the flank angles differ and the joint will not hold pressure. Use an adapter.
| Thread | TPI | Major Ø (mm) | Major Ø (in) | Flank angle | Taper |
|---|---|---|---|---|---|
| G1/2 (this thread) | 14 | 20.955 | 0.8250 | 55° | parallel |
| 1/2-14 NPT | 14 | 21.336 | 0.8400 | 60° | 1 in 16 |
Related sizes and tools
Sources: ISO 228-1 (dimensions and sealing principle), ISO 7-1 (taper threads), tap manufacturers’ drill charts. How this site checks its data. Last reviewed 29 September 2026.