G3/4 BSP Thread Dimensions and Tap Drill
G3/4 is the parallel British Standard Pipe thread (BSPP) of nominal size 3/4: 14 threads per inch on a 55° Whitworth form with rounded crests and roots, and a major diameter of 26.441 mm (1.0410 in). The size names the bore of the pipe it was once cut on, so nothing on the thread measures 3/4 inch.
G3/4 is the washing machine and dishwasher inlet hose thread and the usual garden tap thread in Europe.
- Tap drill24.5 mm0.9646 in, 84% thread
- Threads per inch14pitch 1.814 mm
- Major diameter26.441 mm1.0410 in
- Pitch diameter25.279 mm0.9952 in
- Minor diameter24.118 mm0.9495 in
- Thread height1.162 mm0.0457 in
Thread profile
G3/4 dimensions
| Dimension | mm | in |
|---|---|---|
| Major diameter d = D | 26.441 | 1.0410 |
| Pitch diameter d2 = D2 | 25.279 | 0.9952 |
| Minor diameter d1 = D1 | 24.118 | 0.9495 |
| 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.
G3/4 tap drill size
Tap drill for G3/4: 24.5 mm (0.9646 in)
This is 0.38 mm over the basic minor diameter of 24.118 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 24.118 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 |
|---|---|---|---|---|
| 24.25 mm | 24.25 | 0.9547 | 94% | |
| 24.5 mmrecommended | 24.50 | 0.9646 | 84% | |
| 24.75 mm | 24.75 | 0.9744 | 73% | |
| 25 mm | 25.00 | 0.9843 | 62% | |
| 25.25 mm | 25.25 | 0.9941 | 51% |
% thread = (major diameter − drill diameter) ÷ (2 × thread height).
G3/4 and the tapered R3/4 (BSPT)
The taper thread of ISO 7-1 (EN 10226), written R3/4 for the male and Rc3/4 or Rp3/4 for the female, has the same 14 threads per inch and the same 55° form as G3/4, 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 R3/4 mates with a taper Rc3/4 or a parallel Rp3/4 female thread. Rp is not the same thing as G: the basic sizes agree but the tolerances do not, so a G3/4 port is not a specified mate for a taper male. The tap drill for the taper internal thread Rc3/4 is 23.2 mm, and for the parallel internal thread Rp3/4 it is 24 mm; both are smaller than the 24.5 mm used for G3/4.
How G3/4 seals
A G3/4 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.
G3/4 against 3/4 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 26.670 mm against 26.441 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 |
|---|---|---|---|---|---|
| G3/4 (this thread) | 14 | 26.441 | 1.0410 | 55° | parallel |
| 3/4-14 NPT | 14 | 26.670 | 1.0500 | 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.