G3/8 BSP Thread Dimensions and Tap Drill
G3/8 is the parallel British Standard Pipe thread (BSPP) of nominal size 3/8: 19 threads per inch on a 55° Whitworth form with rounded crests and roots, and a major diameter of 16.662 mm (0.6560 in). The size names the bore of the pipe it was once cut on, so nothing on the thread measures 3/8 inch.
G3/8 is the thread on angle valves and flexible tap connectors in European plumbing.
- Tap drill15.25 mm0.6004 in, 82% thread
- Threads per inch19pitch 1.337 mm
- Major diameter16.662 mm0.6560 in
- Pitch diameter15.806 mm0.6223 in
- Minor diameter14.950 mm0.5886 in
- Thread height0.856 mm0.0337 in
Thread profile
G3/8 dimensions
| Dimension | mm | in |
|---|---|---|
| Major diameter d = D | 16.662 | 0.6560 |
| Pitch diameter d2 = D2 | 15.806 | 0.6223 |
| Minor diameter d1 = D1 | 14.950 | 0.5886 |
| Pitch P | 1.337 | 0.05263 |
| Thread height h (0.640327 × P) | 0.856 | 0.0337 |
| Crest and root radius r (0.137329 × P) | 0.184 | 0.0072 |
| Height of fundamental triangle H | 1.284 | 0.0506 |
| Property | Value |
|---|---|
| Threads per inch | 19 |
| Flank angle | 55° (27.5° each side) |
| Taper | none (parallel thread) |
Basic dimensions from ISO 228-1 Table 1. Inch values are conversions.
G3/8 tap drill size
Tap drill for G3/8: 15.25 mm (0.6004 in)
This is 0.30 mm over the basic minor diameter of 14.950 mm and leaves about 82% 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 14.950 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 |
|---|---|---|---|---|
| 15.25 mmrecommended | 15.25 | 0.6004 | 82% | |
| 15.5 mm | 15.50 | 0.6102 | 68% | |
| 15.75 mm | 15.75 | 0.6201 | 53% |
% thread = (major diameter − drill diameter) ÷ (2 × thread height).
G3/8 and the tapered R3/8 (BSPT)
The taper thread of ISO 7-1 (EN 10226), written R3/8 for the male and Rc3/8 or Rp3/8 for the female, has the same 19 threads per inch and the same 55° form as G3/8, 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 R3/8 mates with a taper Rc3/8 or a parallel Rp3/8 female thread. Rp is not the same thing as G: the basic sizes agree but the tolerances do not, so a G3/8 port is not a specified mate for a taper male. The tap drill for the taper internal thread Rc3/8 is 14.3 mm, and for the parallel internal thread Rp3/8 it is 15 mm; both are smaller than the 15.25 mm used for G3/8.
How G3/8 seals
A G3/8 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.
G3/8 against 3/8 NPT
The American NPT thread of the same nominal size has 18 threads per inch against 19, a 60° flank angle and an outside diameter of 17.145 mm against 16.662 mm. The parts do not fit each other. Use an adapter.
| Thread | TPI | Major Ø (mm) | Major Ø (in) | Flank angle | Taper |
|---|---|---|---|---|---|
| G3/8 (this thread) | 19 | 16.662 | 0.6560 | 55° | parallel |
| 3/8-18 NPT | 18 | 17.145 | 0.6750 | 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.