BSP Thread Dimensions — Male & Female Reference Guide
Identifying the correct BSP thread size requires measuring either the outside diameter of a male fitting or the inside diameter of a female fitting. The nominal BSP size (e.g. "1/2 inch BSP") does NOT equal the actual measurement — a 1/2" BSP male thread is actually 20.96 mm OD.
How to identify BSP thread size
- For male threads: measure the outside diameter with calipers
- For female threads: measure the inside diameter
- Match your measurement to the OD/ID column above
- Confirm by counting TPI (threads per inch) with a thread gauge
Frequently Asked Questions
- Why is 1/2" BSP not actually 1/2 inch?
- BSP nominal sizes date back to the internal bore of old iron pipe, not the thread diameter. A 1/2" BSP fitting has a 20.96 mm thread OD.
- Are BSPP and BSP the same?
- Yes — BSPP (British Standard Pipe Parallel) is the same as what is commonly called BSP. It uses parallel (straight) threads that seal on a face washer or O-ring.
- What is BSPT?
- BSPT (British Standard Pipe Taper) has a slight taper similar to NPT. It is less common on farm hydraulics and more often used on water and gas pipework.
- What is the OD of 1/2" BSP thread?
- The outside diameter of a 1/2" BSP male thread is 20.96 mm. The female thread inside diameter is 19.10 mm. Despite the name, neither measurement is anywhere near 12.7 mm (half an inch).
- How do I measure a BSP fitting size without a thread gauge?
- Measure the outside diameter of the male thread with digital calipers and match to the OD column in the chart above. For female fittings, measure the inside diameter and match to the ID column. If you are between sizes, count the threads per inch (TPI) across a 1-inch length to confirm.
- What is the difference between BSP male and female thread dimensions?
- The male thread is measured by its outside diameter (OD) — the widest point of the thread. The female thread is measured by its inside diameter (ID) — the thread root. For any given BSP size, the male OD is always larger than the female ID by approximately 1.8 mm at the smaller sizes and up to 2.2 mm at the larger sizes.
- Are BSP thread dimensions the same for BSPP and BSPT?
- Yes — BSPP and BSPT share identical thread pitch, TPI and nominal OD/ID dimensions for each size. The difference is that BSPT tapers at 1 in 16 along the thread axis, while BSPP is perfectly parallel. This means you can use the same measuring technique and the same reference table for both types.
- What vernier caliper do I need to measure BSP fittings?
- Any digital caliper with a 150mm (6 inch) range and 0.01mm resolution is sufficient for BSP sizes up to 2". For larger sizes (2-1/2" to 3") you need a 200–300mm caliper. Digital calipers are preferable to manual vernier calipers for workshop use — the digital readout eliminates misreading. Stainless steel jaws are more durable than plastic.