How to Measure BSP Threads

BSP stands for British Standard Pipe. BSP thread sizes are common on caravan plumbing fittings, water fittings and some gas fittings. The confusing part is that the BSP size name is not the same as the outside diameter you measure with a ruler or calipers.

Use this guide as a quick way to convert the measured thread diameter into the common BSP size name before ordering a fitting.

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1. What BSP means

BSP sizes are imperial names that originally related to pipe bore size. That is why a fitting called 1/2" BSP does not measure 1/2" across the outside of the thread.

As a rough rule of thumb, the measured outside diameter of a male BSP thread is about 1/4" larger than the BSP size name. For example, a male thread measuring about 1" outside diameter is usually 3/4" BSP.

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2. How to measure a male thread

For a male BSP fitting, measure across the outside of the thread. Calipers are best, but a ruler can usually get close enough for common caravan plumbing sizes.

The male thread may be slightly tapered, so take the measurement across the thread and compare it with the chart below. Do not measure the smooth shank or the hex section of the fitting.

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3. How to measure a female thread

For a female BSP fitting, measure the inside diameter of the threaded opening. This measurement is smaller than the matching male thread outside diameter, so use the female column in the chart rather than the male column.

If you have the old matching male fitting, it is often easier to measure the male thread and use that as the reference.

Print the BSP thread sight guide if you want a template to lay the fitting against.

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4. Quick conversion chart

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5. Before ordering fittings

• Measure the threaded section, not the nut or body of the fitting.
• Check whether you are measuring a male outside diameter or female inside diameter.
• Confirm whether the fitting is for water, sullage, gas or another use.
• Do not assume BSP, compression, flare, NPT and push-fit fittings are interchangeable.
• Gas work should be handled by a licensed gas fitter.

For broader caravan plumbing layout help, see our caravan plumbing system guide. For push-fit plumbing, see how to use John Guest fittings.