BSP thread sizes —
OD, TPI and how
to identify the fitting
A 1/2 inch BSP male thread measures about 20.95 mm across. A 3/4 inch BSP male thread measures about 26.44 mm. This guide helps you identify the thread size first, then keeps the gasket decision separate so you do not order from the wrong number.
What BSP actually means
BSP — British Standard Pipe — is a pipe thread family used in plumbing, heating, compressed air, hydraulics and industrial process pipework. Parallel BSP threads are covered by ISO 228; taper BSP threads are covered by ISO 7.
The nominal size is expressed in inches — 1/4", 3/8", 1/2", 3/4", 1", and so on. These numbers are a historical carry-over from pipe bore sizing. They do not match the physical diameter of the thread. A 1/2" BSP male thread is about 20.95 mm OD. A 3/4" BSP male thread is about 26.44 mm OD.
This mismatch is the reason people mis-order fittings and gaskets. The nominal inch size is the label used in catalogues. The millimetre OD is how you identify what is actually in your hand.
The practical consequence: if you measure a male thread with a calliper and get about 20.95 mm, you are looking at 1/2" BSP — not 20 mm, not 21 mm and not "half an inch across". The measurement identifies the BSP size. It does not automatically define the flat gasket size.
BSPP vs BSPT — two different thread types
BSP covers two thread forms with different sealing methods. Before choosing a gasket, washer, tape or fitting, identify whether the joint is parallel or taper.
- Parallel thread form — no taper
- Because the sides are straight, the thread itself is not normally the seal
- Commonly seals on a face, shoulder or washer arrangement, depending on fitting design
- Also called G thread or Whitworth parallel pipe
- Common on many plumbing and heating fittings in the UK
- Radiator valves, boiler connections, tap connectors and pump unions
- If it is a flat face union, the gasket size comes from the recess, not from the thread OD
- Female thread marked G — e.g. G1/2, G3/4
- Thread tapers at 1:16 along the axis
- Seals on the thread itself with taper interference and suitable sealant
- Also called R thread (male) or Rc thread (female)
- Used where the thread joint itself is specified as the sealing method
- Typically used with suitable thread sealant, not a flat face gasket
- Same thread pitch and nominal sizes as BSPP
- Male thread marked R — e.g. R1/2, R3/4
Flat gaskets appear on BSPP flat-face unions and similar joints where two faces clamp the gasket. BSPT joints seal through the tapered thread and are a different sealing problem. If a joint has a flat gasket, identify the thread size first, then measure the face recess for the actual gasket dimensions.
BSP dimensions — full chart
The table below covers standard BSP sizes from 1/8" to 4". The most common domestic and light commercial sizes — used in plumbing, heating and HVAC — are 3/8", 1/2", 3/4" and 1". Sizes from 1-1/4 inch upward are more common in industrial and commercial pipework.
| Nominal size | TPI | Thread OD (mm) | Thread OD (in) | Pitch (mm) |
|---|---|---|---|---|
| 1/8″ | 28 | 9.73 | 0.383 | 0.907 |
| 1/4″ | 19 | 13.16 | 0.518 | 1.337 |
| 3/8″ | 19 | 16.66 | 0.656 | 1.337 |
| 1/2″ | 14 | 20.95 | 0.825 | 1.814 |
| 3/4″ | 14 | 26.44 | 1.041 | 1.814 |
| 1″ | 11 | 33.25 | 1.309 | 2.309 |
| 11/4″ | 11 | 41.91 | 1.650 | 2.309 |
| 11/2″ | 11 | 47.80 | 1.882 | 2.309 |
| 2″ | 11 | 59.61 | 2.347 | 2.309 |
| 21/2″ | 11 | 75.18 | 2.960 | 2.309 |
| 3″ | 11 | 87.88 | 3.460 | 2.309 |
| 4″ | 11 | 113.03 | 4.450 | 2.309 |
TPI = threads per inch. Thread OD = outside diameter of the male thread.
The table identifies BSP thread size only. It does not specify a flat gasket dimension. Highlighted rows are most common in domestic and light commercial plumbing and heating.
Quick reference — most common sizes
How to measure BSP thread size
You cannot identify a BSP thread by eye alone. Two fittings that look similar may be different sizes. A calliper measurement removes the guesswork.
If you only have the female fitting: measure the inside diameter of the female thread at the opening. Add approximately 1.5–2 mm as a rough field estimate only — not as an exact measurement. This is less accurate — a thread gauge or matching male fitting is more reliable for female-only identification.
BSP vs NPT — the other common thread standard
NPT stands for National Pipe Thread — the common US pipe thread standard. NPT and BSP threads are not compatible. They have different thread angles (60° for NPT, 55° for BSP), different pitch and different taper geometry. An NPT fitting may appear to start in a BSP fitting for a few turns, but it will not seat correctly and should not be treated as a compatible joint.
If you are working with equipment imported from the United States, confirm whether the connection is NPT before ordering BSP fittings or gaskets. Do not force a near-fit thread and hope the sealant will solve it.
Does the BSP size tell you the gasket size?
No — not by itself. BSP size tells you the thread family and nominal size. On a BSPP flat-face joint, the gasket sits inside the fitting recess. That recess is part of the fitting design, not part of the BSP thread chart.
Use the BSP thread size to get into the right family, then measure the actual sealing face:
- Recess outside diameter: this controls the gasket OD.
- Recess inside diameter: this controls the gasket ID and bore clearance.
- Recess depth or removed gasket thickness: this controls whether the gasket will compress correctly.
- Service conditions: water, oil, gas, steam, temperature, pressure and documentation requirements control material choice.
Do not order a flat gasket from thread OD alone. A 3/4" BSP male thread is about 26.44 mm OD, but that does not mean the gasket OD is 26.44 mm. The gasket has to fit the recess in the union or fitting face. For practical gasket ranges, use the separate BSP flat gasket size guide and then measure the actual joint.
Real life examples — what size is that fitting?
These are common examples from typical UK domestic installations — not universal rules. Fitting sizes vary by manufacturer and installation.
Washing machine hose connector: commonly 3/4" BSP (26.4 mm OD). The hose washer is selected for the hose-end seat, not from thread OD alone.
Standard UK radiator valve: commonly 1/2" BSP on the valve body connection (20.9 mm OD). The tail piece connecting to the radiator is usually 1/2" BSP also.
Combi boiler pressure gauge: commonly 1/4" BSP (13.16 mm OD). Small fitting, easy to mistake for 1/8".
Garden tap body: commonly 1/2" BSP (20.9 mm OD) where it connects to the supply pipe. Many UK garden tap outlets use a 3/4" BSP-style hose union thread for hose connectors, while quick-connect hose couplings attach to that outlet with their own connector system.
Central heating pump union: commonly 1-1/2 inch BSP (47.8 mm OD) for many domestic systems. The union gasket must still be matched to the recess in the pump union face.
Quick answers
BSP nominal sizes are labels, not measurements.
A 1/2" BSP fitting is about 20.95 mm across the male thread, not 12.7 mm. Use the calliper measurement to identify the thread size. Then identify the sealing method. If the joint uses a flat gasket, measure the fitting face before ordering the gasket.