The short answer
BSP (British Standard Pipe) sizes are named after the bore of old iron pipe, not the thread you measure, so a 1/2" BSP thread is about 21mm across the outside and a 3/4" about 26.4mm. Measure the outside of a male thread and match it to the chart. Parallel threads (BSPP) seal with a washer or O-ring; tapered threads (BSPT) seal on the thread with PTFE tape or jointing compound.
BSP thread size chart
| BSP size | Outside diameter | Threads per inch | Common uses |
|---|---|---|---|
| 1/8" | 9.7mm | 28 | Gauges, small air vents |
| 1/4" | 13.2mm | 19 | Pressure gauges, air vents, small valves |
| 3/8" | 16.7mm | 19 | Some radiator valves and appliance connections |
| 1/2" | 21.0mm | 14 | Basin and sink taps, radiator valves, shower hoses, toilet fill valves |
| 3/4" | 26.4mm | 14 | Bath taps, washing machine valves and hoses |
| 1" | 33.2mm | 11 | Cylinder and larger valve connections |
| 1 1/4" | 41.9mm | 11 | Commercial pipework |
| 1 1/2" | 47.8mm | 11 | Pump unions and commercial pipework |
| 2" | 59.6mm | 11 | Commercial pipework |
Immersion heaters use their own larger 2 1/4" boss. Brass fittings let you step between sizes: bushes reduce a female thread, nipples join two female threads, and sockets join two male ones.
How to measure a BSP thread
- Measure across the outside of a male thread with callipers — or the inside of a female thread and add a little.
- Match it to the nearest outside diameter in the chart, not to the size name.
- If two sizes are close, count the threads over 1 inch (25.4mm) to confirm.
The easiest way is to bring the part to the counter — see what to bring to the trade counter.
Parallel (BSPP) or taper (BSPT)?
| Parallel (BSPP, "G") | Taper (BSPT, "R" and "Rc") | |
|---|---|---|
| Shape | Same diameter along the thread | Narrows along the thread |
| Where it seals | On a washer, O-ring or flat face | On the thread itself |
| How to seal | Fibre or rubber washer; no tape needed | PTFE tape or jointing compound |
| Typical use | Tap connectors, hoses, unions | Fittings screwed into valves and pipework |
Wind PTFE tape clockwise as you look at the end of the male thread, so tightening doesn't unwind it. Use gas-rated tape or compound on gas.
Joining metric pipe to BSP threads
Pipe is metric, but threads on taps, valves and appliances are BSP, so adaptors combine the two: a 15mm × 1/2" fitting joins 15mm pipe to a 1/2" thread, and 22mm × 3/4" is the other common pair. They come in compression, solder and push-fit, male or female.
BSP fittings we stock
| Fitting | Lines | From |
|---|---|---|
| Brass bushes | 46 | £0.63 |
| Brass nipples | 21 | £0.96 |
| Jointing compounds | 22 | £2.90 |
Questions
- What is 1/2 inch BSP in mm?
- About 21mm across the outside of the male thread (20.96mm), with 14 threads per inch. The name comes from old iron pipe bore, not the thread.
- Is 3/4 BSP the same as 22mm?
- No. 3/4" BSP is a thread about 26.4mm across; 22mm is a pipe size. A 22mm × 3/4" adaptor joins the two.
- Do I need PTFE tape on BSP threads?
- On taper threads, yes — PTFE tape or jointing compound. Parallel threads seal on a washer, so tape isn't needed and can stop the washer seating.
- Can I connect BSP to NPT?
- Not directly — the threads differ and will leak. Use a proper BSP-to-NPT adaptor.