Why a half-inch pipe is not half an inch

A label, not a dimension

Nominal pipe size began as the internal bore of thick-walled iron pipe. Wall thicknesses shrank as materials improved, the bores changed, and the labels stayed put. A half-inch BSP fitting now measures about 20.96 mm across its outside diameter — nowhere near 12.7 mm.

Metric plumbing is literal

European copper and plastic pipe is named after its true outside diameter: 15 mm pipe measures 15 mm. That is clearer, and it is exactly why the two systems will not screw together. An adapter is a requirement, not a convenience.

Identifying a fitting properly

1. Measure the outside diameter with a caliper.
2. Count threads per inch with a thread gauge, or measure the pitch if metric.
3. Check the thread angle — 55 degrees is BSP, 60 degrees is NPT.
4. Look up the combination rather than trusting the box.

BSP and NPT are not interchangeable

They engage far enough to feel right and seal badly. The thread angles differ, so the flanks never mate properly. A joint made this way often holds during a pressure test and weeps a fortnight later.

The one exact figure

Whatever the labels do, an inch is exactly 25.4 mm. When you are comparing real measurements rather than trade names, that conversion never varies.

What the number once meant

In the nineteenth century, wrought-iron pipe of a given trade size really did have an internal bore close to its label. Wall thicknesses were generous because the material and the manufacturing tolerances demanded it. As steel improved, walls became thinner while outside diameters stayed fixed — because that is what the threading dies and fittings were built around.

The result is a size designation that describes neither the bore nor the outside diameter of a modern pipe. It identifies a family of compatible parts, nothing more.

Common sizes and their real dimensions

Note that the thread count changes with size, which is why a thread gauge identifies a fitting more reliably than a ruler alone.

Parallel versus tapered

BSP threads come in two forms that look nearly identical. BSPP is parallel and seals on a washer or O-ring at the face. BSPT is tapered and seals on the threads themselves, usually with sealant or PTFE tape. Mating a tapered male into a parallel female works in some combinations and leaks in others, and the failure is typically slow enough to appear after the job is signed off.

NPT, the American standard, is tapered with a 60-degree flank angle against BSP's 55 degrees. The two engage far enough to feel correct and never seal properly.

Identifying a fitting in four steps

1. Measure the outside diameter of a male thread, or the inside diameter of a female one, with a caliper.
2. Count threads across one inch with a gauge, or measure the pitch in millimetres.
3. Check whether the thread is parallel or tapered by measuring the diameter at both ends of the thread.
4. Match those three facts against a standards chart rather than guessing from the packaging.

Metric transition fittings

Where metric copper meets imperial threads, use a purpose-made adapter with the correct seal on each side. Compression fittings sized for 15 mm pipe will grip half-inch imperial pipe badly, because 12.7 mm nominal imperial tube has a different actual outside diameter — the olive deforms unevenly and the joint weeps under thermal cycling.

The takeaway

Treat the trade size as a part number and measure the real dimensions when it matters. An inch is exactly 25.4 mm; a half-inch fitting is exactly nothing you can measure.