How are plumbing threads measured - BSP or not BSP!

How are plumbing threads measured - BSP or not BSP!

This catches many people out, with people often receiving a plumbing product larger than they expected. This is because most plumbing products are measured using BSP (British Standard Pipe) thread dimensions which measure approximately 1/4 inches larger than that stated. British Standard pipe was used back in the day and was the size of thread on a fitting used to connect to a pipe with the inside diameter stated in BSP terms. It is largely irrelevant today as a concept of measuring pipe as most modern metric pipe is simply stated using the outside diameter of the pipe. However, most manufacturers still use BSP thread dimensions for all plumbing threads, some conversion examples are shown below

  • A  3/8 Inch BSP thread measures approximately 5/8 inch in diameter
  • A 1/2 Inch BSP thread measures approximately 3/4 inch in diameter
  • A 3/4 Inch BSP thread measures approximately 1 inch in diameter
  • A 1 Inch BSP thread measures approximately 1 1/4 inches in diameter
  • A 1 1/4 Inch BSP thread measures approximately 1 1/2 inches in diameter
  • A 1 1/2 Inch BSP thread measures approximately 1 3/4 inches in diameter.....etc

As an additional observation many DIY shops import bathroom fittings from mainland Europe where a standard domestic water pipe uses a 3/8 inch BSP connection, as opposed to the slightly larger UK 1/2 inch BSP equivalent. For instance a toilet fill valve may require a 15mm to 3/8 inch BSP flexible tap connector to connect it to our 15mm pipework

We supply many different plumbing fittings that use BSP threads with a few examples linked below

Flexible Tap Connectors

Brass Threaded Plumbing Fittings

Plumbing Valves

1st Mar 2020 Matt

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