How to Measure BSP Fittings

The one rule that trips people up: a BSP fitting’s stated size is not its measured size. For a male (external) fitting, the nominal BSP size equals the measured outside diameter minus roughly 1/4 inch (0.25″). Get that backwards and every subsequent step — matching it to a chart, ordering a replacement — starts from a wrong number.

Step-by-Step Measurement

1. Determine parallel (BSPP) or tapered (BSPT). Run your fingers or a caliper along the thread length. If the diameter stays constant, it’s BSPP. If it narrows progressively toward one end, it’s BSPT.
2. Measure the diameter with calipers. For a male fitting, measure the outside diameter (OD) across the crest of the threads. For a female fitting, measure the inside diameter (ID) of the threaded bore.
3. Subtract about 1/4 inch (0.25″) from the measurement to get the nominal size. Example: a measured OD of 2.25″ corresponds to a nominal 2″ BSP size (2.25 − 0.25 = 2.00). A measured OD of about 1″ corresponds to a nominal 3/4″ BSP size.
4. Count the threads per inch (TPI) to cross-check. Use a thread pitch gauge, or count the number of thread crests over a measured 1/4″ span and multiply by four. This step matters because more than one nominal size can produce a similar caliper reading — TPI is what confirms you’ve landed on the right one.
5. Confirm against a standard BSP size chart rather than relying on the calculation alone, especially for less common sizes.

Reference Chart (Common Sizes)

These figures are drawn from industry measurement references, not quoted directly from the BS 21 / ISO 228 standard text — treat them as a practical starting point for identification, and confirm against the standard or a supplier’s spec sheet before ordering or machining to size.

Worked Example

Say you measure a male fitting at 21 mm outside diameter and count 14 threads per inch. Cross-referencing the table above: 21.0 mm OD and 14 TPI both point to a 1/2″ BSP fitting. If only one of the two numbers matched, that would be a sign to re-measure — a correct identification should agree on both diameter and pitch, not just one.

Before You Order or Adapt

Confirming BSP size is only half the job if you’re connecting to non-BSP equipment. Check whether you’re actually dealing with BSPP (parallel, face-sealed) or BSPT (tapered, thread-sealed) — see What Is BSPP? British Standard Pipe Parallel Explained for that distinction — and don’t assume a BSP fitting will mate with an NPT part of the same TPI; see NPT vs. BSPP: Why They Don’t Interchange for why that assumption fails even when the thread counts line up.

Related reading: What Is BSPP? British Standard Pipe Parallel Explained · NPT vs. BSPP: Why They Don’t Interchange

Danny Ni Engineering & Mechanical Systems Writer

Danny Ni is an engineering-focused technical writer at SYZ Machine, specializing in mechanical components, linkage systems, and real-world application engineering. His work covers aftermarket vehicle parts, industrial joints, and mechanical principles, translating complex engineering concepts into practical insights for engineers, fabricators, and industry buyers.

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