BSP Thread Sizes Explained

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BSP, or British Standard Pipe, can be confusing, especially when measuring existing fittings for replacement.

Many assume that the BSP thread size refers to the outer diameter of the male threads. This is incorrect. To find the thread size, subtract ¼” (0.25”) from the outer measurement. Use the chart below for accurate sizing.

Thread Type

BSPP Parallel (Straight) Threads - These threads run parallel to the centreline and typically seal with a washer or o-ring.

BSPT Taper Threads - These threads are angled relative to the centreline and seal through the engagement of two mating threads.

Thread per Inch

To confirm a fitting is BSP, count the number of threads over a ¼” length and multiply by 4 to determine the threads per inch.

For example, if there are 7 thread crests in ¼”, then 7 (thread crests) x 4 (conversion to inch) = 28 (Threads per Inch).

A screw pitch gauge can also be used for verification.

Typical BSP sizes and their uses include:

• ¼” - Compressed air connectors.
• ⅜” - Central heating.
• ½” - Household taps, radiator connections, and shower hoses.
• ⅝” - Old outside taps.
• ¾” - Bath taps, outdoor taps, and washing machines.
• ⅞” - Outdoor taps.
• 1” - Boilers and cylinders.
• 1 ¼” - Sinks.
• 1-1/2 Inch BSP - Kitchen and bathroom wastes, toilet syphons, and central heating pumps.
• 2-1/4 Inch BSP - Immersion water heaters.

Male

Female

How to measure BSP Thread Sizes

• First, use a ruler or vernier to measure across the thread O/D for male threads and the I/D for female threads.
• Place the ruler across the threads to measure the diameter. Note that this is not the BSP thread size.
• Once you have your measurement, for example, approximately 1”, this will not be the BSP size. Referring to the table below, you will actually need a ¾” BSP thread size.