BSP Sizes
Radiator Thread Sizes

THREAD SIZES

BSP sizes do not match up with what you measure on a ruler.

A commonly made mistake when purchasing pipe fittings is measuring the thread width in millimetres (mm) and then converting this value into inches. This will give an incorrect thread size and therefore lead you to buying the wrong fitting.

Pipe fitting sizes are based on the internal bore of the passage which carries the air, gas or liquid. The thread size will be larger than this.

Approximately, to determine the thread size:

Measure the thread’s outer diameter (OD) in inches, subtract ¼” from the OD measurement.

So if you measure the thread OD at 1” then it is likely to be a ¾” fitting.

BSP Fitting Size Measured Male Thread OD Measured Female Thread OD
mm inch mm inch
1/8" 9.5 3/8" 8.4 0.33
1/4" 12.8 1/2" 11.2 0.44
1/2" 20.4 3/4" 18.3 0.72
3/4" 25.9 1" 23.9 0.94
1" 32.6 1 1/4" 29.7 1.17
1.1/4" 41.1 1 5/8" 38.6 1.52
1.1/2" 47.0 1 7/8" 44.5 1.75
2" 58.6 2 3/8" 56.4 2.22

Measure Right first time

We’ve made a handy PDF that you can print out at home to use as a gauge when measuring threads. Download our BSP Thread Size Template, print it onto A4 paper at 100% scale and follow the instructions. It includes circles that are sized for various BSP threads as well as a handy cut-off tape measure that wraps around the pipe if the ends aren’t accessible.

The nominal size refers not to the outside of the pipe, nor even to the outside of the thread, but to its internal bore – the inside of the tube where the fluid flows. It makes sense when sizing pipes for flow rate but it makes things confusing when measuring for replacement parts.

Below is a summary of the common BSP thread sizes in used with cast iron radiators and their corresponding actual outside diameters in millimetres and inches. The actual outside diameter is what you can measure when looking at a valve tail, a threaded nipple, reducing bush or an opening in the radiator.

G & R Thread Types

G is the international standard shorthand for BSPP  – BSP Parallel threads. R is the standard for BSPT – BSP Tapered. Parallel (G) threads are used where the seal is made on a mating face, like inside a cast iron radiator with the aid of a gasket.

Tapered fittings achieve a tight thread-to-thread seal with the help of a little sealant such as PTFE tape or hemp and Boss White.

The difference between BSP and NPT

NPT – National Pipe Thread – is the US standard. Most of the rest of the world uses BSP – British Standard Pipe.

NPT has flattened peaks and troughs where BSP is rounded. NPT has a thread angle of 60º where BSP is 55º.

At certain sizes, NPT and BSP share the same number of threads per inch – at ½” and ¾” – but at all other sizes the two are quite different. The table below lists the TPI for both NPT and BSP at sizes commonly used with domestic radiators.

Nominal pipe size NPT BSP
Threads per inch Threads per inch
1/8" 27 28
1/4" 18 19
3/8" 18 19
1/2" 14 14
3/4" 14 14
1" 11-1/2 11
1-1/4" 11-1/2 11
1-1/2" 11-1/2 11
2" 11-1/2 11
2-1/2" 8 11