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Pipe and thread sizes: nominal inches versus real millimetres

A pipe sold as half an inch does not measure 12.7 mm anywhere on it. Plumbing uses nominal sizes — labels that once described a dimension and now simply identify a standard. Understanding the difference between the label and the measurement is what stops you buying fittings that will not seal.

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Nominal size, bore and outside diameter

Nominal pipe size began as the internal bore in inches, measured on the thick-walled iron pipe of the nineteenth century. Wall thicknesses changed as materials improved, but the labels stayed, so the number is now a designation rather than a dimension.

A half-inch BSP thread, for example, has an outside diameter of about 20.96 mm. The half inch refers loosely to the bore of the original pipe, not to anything you can put a caliper on. This is why a thread gauge is worth owning: measuring the outside diameter and counting threads per inch identifies a fitting far more reliably than guessing from the label.

Metric plumbing takes the opposite approach. Copper and plastic pipe in Europe is specified by actual outside diameter — 15 mm pipe really is 15 mm across. That makes it straightforward on its own terms and awkward at the boundary, where adapters exist precisely because 15 mm and half-inch systems do not simply screw together.

Key facts

Nominal is a label

A half-inch pipe measures neither 12.7 mm bore nor 12.7 mm outside diameter.

BSP half inch

The outside diameter is roughly 20.96 mm with fourteen threads per inch.

Metric is literal

European copper and plastic pipe is named after its true outside diameter.

Exact conversion

One inch is exactly 25.4 mm — that part never varies.

Threads per inch

TPI is counted over an inch of thread, so it stays imperial even on metric jobs.

Adapters are normal

Imperial and metric systems need a transition fitting; forcing them risks a slow leak.

How to get it right

1

Measure, do not guess

Take the outside diameter with a caliper rather than trusting the label.

2

Count the threads

Use a thread gauge to establish threads per inch or metric pitch.

3

Convert for comparison

Multiply inches by 25.4 to sit both figures in the same units.

4

Match the standard

Confirm whether the fitting is BSP, NPT or metric before ordering.

Questions people ask

Exactly 12.7 mm as a length. As a pipe size, half inch is a nominal designation and the fitting measures around 21 mm outside diameter.

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