Pipe & thread size converter
BSP thread sizes in millimetres, imperial copper against metric fittings, steel pipe bore to outside diameter, and an identifier for the thread or pipe in your hand.
Identify a thread
Millimetres to inches
BSP thread sizes
BSP sizes are named after the bore of the pipe they were cut on, not the thread, so a ½" BSP thread is about 21 mm (0.825") across, not half an inch. The inches column is the real outside diameter, for anyone converting BSP to imperial or metric. Parallel (BSPP, "G") and taper (BSPT, "R") share these diameters and pitches; the difference is in how they seal.
| Size | Major dia. mm | Major dia. in | TPI | Pitch mm | Tapping drill mm | Where you meet it |
|---|---|---|---|---|---|---|
| ⅛" BSP | 9.73 | 0.383 | 28 | 0.907 | 8.8 | Pressure gauges, small valves |
| ¼" BSP | 13.16 | 0.518 | 19 | 1.337 | 11.8 | Gauges, air fittings, drain cocks |
| ⅜" BSP | 16.66 | 0.656 | 19 | 1.337 | 15.25 | Air tools, small valves, gas cooker bayonet |
| ½" BSP | 20.95 | 0.825 | 14 | 1.814 | 19.0 | Basin and kitchen taps, radiator valves, shower hoses and diverter outlets |
| ¾" BSP | 26.44 | 1.041 | 14 | 1.814 | 24.5 | Bath taps, shower bar mixer and diverter inlets, washing-machine valves, garden taps |
| 1" BSP | 33.25 | 1.309 | 11 | 2.309 | 30.75 | Stopcocks, cylinder connections, pumps |
| 1¼" BSP | 41.91 | 1.650 | 11 | 2.309 | 39.5 | Waste traps (basin), pumps |
| 1½" BSP | 47.80 | 1.882 | 11 | 2.309 | 45.25 | Waste traps (bath, sink), pumps |
| 2" BSP | 59.61 | 2.347 | 11 | 2.309 | 57.0 | Larger valves, commercial |
| 2½" BSP | 75.18 | 2.960 | 11 | 2.309 | 72.5 | Commercial |
| 3" BSP | 87.88 | 3.460 | 11 | 2.309 | 85.0 | Commercial |
| 4" BSP | 113.03 | 4.450 | 11 | 2.309 | 110.0 | Commercial |
Major diameter is the outside of the male thread (ISO 7-1 / ISO 228-1). A female thread measures a little under this at the crests: about 24 mm inside for ¾" BSP and 19 mm for ½" BSP. Tapping drill sizes are the usual chart values for Rc and G taps.
BSPP or BSPT?
- BSPP (G, parallel) seals on a fibre washer, O-ring or cone at the face. Taps, tap connectors, hoses, radiator valve tails, cylinder bosses.
- BSPT (R, taper) seals on the thread with PTFE tape or compound. Malleable iron fittings, steel pipe, many drain cocks and gauges.
- A taper male will screw into a parallel female and often into a taper one, but parallel into taper is a poor seal. If it has a washer, it's parallel.
- NPT (American) looks similar at ½" and ¾" but has a 60° thread form and different TPI (14 on ½" and ¾", so those two almost fit, and then leak).
Identify a pipe
Copper pipe: imperial to metric
Metric copper (BS EN 1057) is named by outside diameter. Imperial copper (BS 659, pre-1970s and still in older houses) was named by bore, so its outside diameters fall just under the metric sizes. That gap decides whether a modern fitting will go on.
| Metric | Imperial | Imperial OD mm | Fits metric fittings? |
|---|---|---|---|
| 8 mm | ¼" | 7.9 | Yes, with an imperial olive |
| 10 mm | ⅜" | 11.1 | No: ⅜" is larger than 10 mm. Use a ⅜" × 10 mm adaptor |
| 15 mm | ½" | 14.3 | Yes: 15 mm compression with a ½" olive; 15 mm solder fittings fit directly |
| 22 mm | ¾" | 20.6 | No: ¾" is a loose fit in 22 mm. Use a 22 mm × ¾" adaptor or ¾" olive |
| 28 mm | 1" | 27.0 | Yes: 28 mm compression with a 1" olive; 28 mm solder fittings usually fit |
| 35 mm | 1¼" | 33.3 | No: use an adaptor |
| 42 mm | 1½" | 39.7 | No: use an adaptor |
| 54 mm | 2" | 52.4 | No: use an adaptor |
Plastic push-fit (PEX, polybutylene) uses the metric sizes: 10, 15, 22 and 28 mm. Chrome-plated copper is the same size as plain.
Steel pipe: nominal bore to outside diameter
Steel tube (BS EN 10255, the old BS 1387) is named by nominal bore, the DN number is the same size in metric, and the outside diameter is what a BSP thread is cut on. This is why ½" pipe, ½" BSP and DN15 all describe the same thing and none of them measure half an inch.
| Nominal bore | DN | Outside dia. mm |
|---|---|---|
| ⅛" | DN6 | 10.2 |
| ¼" | DN8 | 13.5 |
| ⅜" | DN10 | 17.2 |
| ½" | DN15 | 21.3 |
| ¾" | DN20 | 26.9 |
| 1" | DN25 | 33.7 |
| 1¼" | DN32 | 42.4 |
| 1½" | DN40 | 48.3 |
| 2" | DN50 | 60.3 |
| 2½" | DN65 | 76.1 |
| 3" | DN80 | 88.9 |
| 4" | DN100 | 114.3 |
Waste and soil pipe sizes
| Nominal | Imperial | Used for |
|---|---|---|
| 32 mm | 1¼" | Basin, hand basin |
| 40 mm | 1½" | Bath, shower, kitchen sink |
| 50 mm | 2" | Multiple appliances, long runs |
| 110 mm | 4" | Soil and vent pipe |
| 21.5 mm | ¾" | Overflow |
Push-fit (BS 5254) and solvent-weld (BS EN 1329) waste pipe are both sold as 32 and 40 mm but differ in outside diameter by a millimetre or two, so they do not interchange. Join them with a universal (multifit) fitting.
Common fittings and their threads
| Fitting | Thread |
|---|---|
| Basin, kitchen and monobloc tap tails | ½" BSP (monobloc often 10 mm or ⅜" flexi) |
| Bath taps | ¾" BSP |
| Shower bar mixer and diverter valve inlets | ¾" BSP (measures about 24 mm inside) |
| Shower diverter outlet, riser rail and hose, both ends | ½" BSP |
| Washing machine and dishwasher valves and hoses | ¾" BSP |
| Outside garden tap and hose connectors | ¾" BSP (some ½") |
| Radiator valve tail into radiator | ½" BSP |
| Basin waste and trap | 1¼" BSP |
| Bath, shower and sink waste and trap | 1½" BSP |
| Cylinder immersion heater boss | 2¼" BSP |
| Gas cooker bayonet | ½" (12 mm) or ⅜" |
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Common questions
A ½" BSP thread is about 21 mm across the outside (20.955 mm major diameter) with 14 threads per inch. The ½" refers to the bore of the pipe it was designed for, not the thread, which is why a ½" BSP fitting looks nearer ¾". Basin taps, radiator valves and shower hoses all use it.
About 26.4 mm outside diameter, 14 threads per inch. Bath taps, washing-machine and dishwasher valves and outside garden taps use ¾" BSP.
A female thread that measures about 24 mm across the inside is ¾" BSP: the male that fits it is 26.4 mm outside, or 1.041" in inches. That is the usual inlet size on a bar mixer or a two- or three-way diverter valve, with the outlets to the hose and riser at ½" BSP (21 mm outside, 19 mm inside). A 24 mm measurement is not a 24 mm metric thread, and it is not "about an inch": 24 mm is 0.945", but BSP names do not match the measured size. Type 24 into the identifier above as a female thread to see the match.
BSPP (G) is a parallel thread that seals on a washer or O-ring at the face. BSPT (R) is a taper thread that seals on the thread itself with PTFE tape or jointing compound. They share the same diameters and pitch, so a taper male will start in a parallel female, but it will not seal reliably. Taps, hoses and most plumbing fittings are parallel; iron pipe fittings are taper.
Usually. Imperial ½" copper is 14.3 mm outside, so 15 mm solder fittings fit, and 15 mm compression fittings work with a ½" imperial olive. The one that catches people is ¾": at 20.6 mm it is too loose for 22 mm fittings, so you need a 22 × ¾ adaptor or a ¾" olive. 1" (27 mm) fits 28 mm fittings with an imperial olive.
Measure the outside diameter of a male thread, or the inside diameter of a female, with calipers, then count the threads in one inch (25.4 mm). Match the diameter in the table: a ½" BSP male is about 21 mm with 14 TPI. The identifier on this page does the matching from the diameter.
No. Both are sold as 32 mm and 40 mm but the actual outside diameters differ by a millimetre or two between the push-fit (BS 5254) and solvent-weld (BS EN 1329) systems, and between brands. They are not interchangeable; use a universal (multifit) fitting to join them.