G1/4 BSP Thread Dimensions and Tap Drill
G1/4 is the parallel British Standard Pipe thread (BSPP) of nominal size 1/4: 19 threads per inch on a 55° Whitworth form with rounded crests and roots, and a major diameter of 13.157 mm (0.5180 in). The size names the bore of the pipe it was once cut on, so nothing on the thread measures 1/4 inch.
G1/4 is the usual thread for pneumatic fittings, pressure gauges and PC water-cooling parts.
- Tap drill11.8 mm0.4646 in, 79% thread
- Threads per inch19pitch 1.337 mm
- Major diameter13.157 mm0.5180 in
- Pitch diameter12.301 mm0.4843 in
- Minor diameter11.445 mm0.4506 in
- Thread height0.856 mm0.0337 in
Thread profile
G1/4 dimensions
| Dimension | mm | in |
|---|---|---|
| Major diameter d = D | 13.157 | 0.5180 |
| Pitch diameter d2 = D2 | 12.301 | 0.4843 |
| Minor diameter d1 = D1 | 11.445 | 0.4506 |
| 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.
G1/4 tap drill size
Tap drill for G1/4: 11.8 mm (0.4646 in)
This is 0.36 mm over the basic minor diameter of 11.445 mm and leaves about 79% 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 11.445 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 |
|---|---|---|---|---|
| 11.6 mm | 11.60 | 0.4567 | 91% | |
| 11.7 mm | 11.70 | 0.4606 | 85% | |
| 11.75 mm | 11.75 | 0.4626 | 82% | |
| 11.8 mmrecommended | 11.80 | 0.4646 | 79% | |
| 11.9 mm | 11.90 | 0.4685 | 73% | |
| 12 mm | 12.00 | 0.4724 | 68% |
% thread = (major diameter − drill diameter) ÷ (2 × thread height).
G1/4 and the tapered R1/4 (BSPT)
The taper thread of ISO 7-1 (EN 10226), written R1/4 for the male and Rc1/4 or Rp1/4 for the female, has the same 19 threads per inch and the same 55° form as G1/4, 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 R1/4 mates with a taper Rc1/4 or a parallel Rp1/4 female thread. Rp is not the same thing as G: the basic sizes agree but the tolerances do not, so a G1/4 port is not a specified mate for a taper male. The tap drill for the taper internal thread Rc1/4 is 10.85 mm, and for the parallel internal thread Rp1/4 it is 11.5 mm; both are smaller than the 11.8 mm used for G1/4.
How G1/4 seals
A G1/4 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.
G1/4 against 1/4 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 13.716 mm against 13.157 mm. The parts do not fit each other. Use an adapter.
| Thread | TPI | Major Ø (mm) | Major Ø (in) | Flank angle | Taper |
|---|---|---|---|---|---|
| G1/4 (this thread) | 19 | 13.157 | 0.5180 | 55° | parallel |
| 1/4-18 NPT | 18 | 13.716 | 0.5400 | 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.