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Measure the thread and read off the size. Female fittings are measured across the inside diameter, male fittings across the outside diameter of the thread.
Thread dimensions
| BSP Thread Size | Female Inside Diameter | Male Outside Diameter |
|---|---|---|
| 1/8" | 8.6mm | 9.7mm |
| 1/4" | 11.5mm | 13.2mm |
| 3/8" | 15.0mm | 16.7mm |
| 1/2" | 18.6mm | 21.0mm |
| 5/8" | 20.6mm | 23.0mm |
| 3/4" | 24.1mm | 26.4mm |
| 1" | 30.3mm | 33.3mm |
| 1 1/4" | 39.0mm | 41.9mm |
| 1 1/2" | 44.9mm | 47.8mm |
| 2" | 56.7mm | 59.6mm |
| 2 1/2" | 72.2mm | 75.2mm |
| 3" | 84.9mm | 87.9mm |
Scroll the table sideways to see all columns.
Measuring tips
BSP sizes are named after the old pipe bore, not the thread itself — a 1/2" BSP thread measures about 21mm across, not 12.7mm. Measure with calipers across the crests of a male thread, or across the roots inside a female one. If a fitting sits between two sizes, send us a photograph and the measurement before ordering: parallel (BSPP) and tapered (BSPT) threads share these diameters but do not seal the same way.
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