How to Measure BSP Threads

BSP (British Standard Pipe)

BSP threads are the most common and can be used for gas and water. The sizes are imperial.

When used for water the most common one you will know around the home is on your garden tap. Most garden hose fittings come with a 3/4" BSP to 1" BSP adapter.

The size quoted for threads is indicative of the internal bore size, NOT the diameter of the thread so you will need to convert your measurement using the chart below.

A quick rule of thumb is to subtract 1/4 inch from your thread measurement to calculate actual thread size, EG a thread with an outer diameter of 1 inch would have a thread size of 3/4 inch.

Click here to print a Thread Sight Guide to lay your thread on.

Above: If you have an existing thread that looks like those above, you need to measure the outside of the male thread as shown. The male thread is generally tapered so that it gets tighter when screwed in.

9.5mm measured OD is Male BSP 1/8"
13mm measured OD is Male BSP 1/4"
16mm measured OD is Male BSP 3/8"
21mm measured OD is Male BSP 1/2"
26mm measured OD is Male BSP 3/4"
33mm measured OD is Male BSP 1"

Above: If it is a female fitting, measure the inside to determine the actual name. Measured sizes are shown as approx. mm

8mm measured ID is Female BSP 1/8"
11mm measured ID is Female BSP 1/4"
14mm measured ID is Female BSP 3/8"
19mm measured ID is Female BSP 1/2"
24mm measured ID is Female BSP 3/4"
31mm measured ID is Female BSP 1"