BSP Thread Sizes – A constant source of frustration for our customers

It should be simple.

You measure something, and that is the size.


Not if you are measuring a thread, especially if it is a B.S.P. (Bristish Standard Pipe) thread……The size was originally based on the inner diameter measured in inches of a steel tube for which the thread was intended. This often causes confusion, as people think the size refers to the outside diameter of the male thread.

For example, the most common BSP Size for UK Immersion Heaters is 2¼” BSP.
However, if you measure roughly across the threads to check the diameter, it looks closer to 2½”, 2.58″ or 65.71mm, to be precise. But, how can you be precise when you have just struggled to remove an old immersion heater and you might be using a tape measure to try to gauge the thread?

If you ask for a 2½” BSP immersion heater to be made, it will arrive with a thread size of almost 2.96″ or 75.18mm, which will be really annoying as there is no way it will fit, it will be the thread size you asked for but not the size you need. It is the same for all other BSP threads so caution is required.

Here at TPFay will try our best to guide you when you ask for a particular BSP size on a heater you would like us to make for you. We may seem to ask you to check the size you need, to the point of annoyance, but we are trying to guide you. We have encountered so many customers who have unwittingly ordered an immersion heater with the wrong size, it can be an expensive and frustrating experience.

Below is a table of common BSP sizes we use for TPFay Immersion Heaters and Element Fittings. Because we use it all the time it is simple for us, but we know it is less than straightforward if you are unfamiliar with the sizes, so was have a handy tip to help you identify the thread size you need if you are trying to measure around a male thread, it is almost impossible with a tape or desk rule.

  1. Cut a strip of paper that you can wrap around the thread you want to measure.
  2. Draw a Line across it with a pen or pencil.
  3. Wrap the paper strip once around the thread and make a second mark where the paper overlaps the first mark.
  4. Unwrap the paper and measure the distance between the marks. This should correspond to one of the measurements on C=Circumference (C)  mm  above.

If you have any difficulty, give us a call 03703 505058.