Conversion table thread sizes

BSP Threads (British Standard Pipe) have a coding system where the thread size does not correspond to thread code × 2.54 cm (the inch).

This coding originates from the pipe size (gas pipe) for which the thread was originally developed. Measuring the thread and converting it directly will therefore not give you the correct result.

The table below shows the dimensions of internal and external parallel BSP threads (BSPP), often indicated with a G, e.g. G3/8”. BSPP male thread can only be mounted into BSPP female thread.

Tapered BSPT thread is often indicated with an R, e.g. R1/2”. BSPT male thread can be mounted into both parallel and tapered female threads. The best way to measure a tapered thread fitting is at half the thread length.

There are many types of parallel and tapered threads. Differences lie in thread form (flank angle, thread height, pitch, external and internal diameters, and others). This ensures you always know which type of thread you are dealing with for your installation.

Tip: use a thread pitch gauge.