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BSPT Thread Chart
Complete BSPT (R) pipe thread dimensions per ISO 7/1 / BS 21 — major, minor and pitch diameter, TPI, metric tap drill size and engagement length.
BSPT is the tapered British Standard Pipe thread, marked R (for example R1/2). It uses a 55° Whitworth thread form against NPT's 60°, which is what makes the two incompatible despite sharing nominal sizes. Its parallel sibling BSPP is marked G and seals on a washer rather than on the threads.
| Size | TPI | Major dia | Major (mm) | Minor dia | Pitch dia | Tap drill | Hand-tight |
|---|---|---|---|---|---|---|---|
| 1/8″ | 28 | 0.383″ | 9.73 | 0.337″ | 0.3566″ | 8.6 mm | 0.148″ |
| 1/4″ | 19 | 0.518″ | 13.16 | 0.46″ | 0.4868″ | 11.5 mm | 0.209″ |
| 3/8″ | 19 | 0.656″ | 16.66 | 0.598″ | 0.6248″ | 15 mm | 0.224″ |
| 1/2″ | 14 | 0.825″ | 20.95 | 0.75″ | 0.7842″ | 18.6 mm | 0.291″ |
| 3/4″ | 14 | 1.041″ | 26.44 | 0.966″ | 1.0003″ | 24.1 mm | 0.311″ |
| 1″ | 11 | 1.309″ | 33.25 | 1.214″ | 1.2572″ | 30.5 mm | 0.409″ |
| 1-1/4″ | 11 | 1.65″ | 41.91 | 1.555″ | 1.5982″ | 39.2 mm | 0.43″ |
| 1-1/2″ | 11 | 1.882″ | 47.8 | 1.787″ | 1.8302″ | 44.8 mm | 0.43″ |
| 2″ | 11 | 2.347″ | 59.61 | 2.252″ | 2.2952″ | 56.7 mm | 0.449″ |
| 2-1/2″ | 11 | 2.96″ | 75.18 | 2.865″ | 2.9082″ | 72.2 mm | 0.449″ |
| 3″ | 11 | 3.46″ | 87.88 | 3.365″ | 3.4082″ | 84.8 mm | 0.449″ |
| 4″ | 11 | 4.46″ | 113.28 | 4.365″ | 4.4082″ | 109.5 mm | 0.449″ |
All of these threads are tapered 3/4″ per foot on diameter (1:16), which is 1°47′ of included taper. Tap drill sizes are
starting points; material and tap maker both shift them slightly.