Inch & Metric Thread Chart: Understanding Thread Sizes

Metric thread sizes are specified differently than inch thread sizes. Inch sizes are specified using the nominal major diameter and the number of threads per inch.

For example, a 3/8-16 (coarse) thread has a 3/8″ nominal major diameter and 16 threads per inch. In contrast, a 3/8-24 (fine) thread has the same nominal major diameter and 24 threads per inch.

Understanding Inch & Metric Thread Callouts

Metric thread callouts are fundamentally different because the thread pitch is specified instead of threads per inch or per mm. The thread pitch is the distance between each thread in mm. For example, an M10x1.5 (coarse) thread has a 1.5mm pitch, while an M10x1.25 (fine) thread has a 1.25mm pitch.

To simplify metric thread callouts, the international (ISO) standard for metric threads eliminates the pitch callout on coarse threads.

For example, the thread “M10” automatically means a coarse pitch, while any added pitch callout, such as M10x1.25, specifies a non-coarse pitch. The ISO standard for measuring thread sizes eliminates the need to memorize pitch callouts. This makes it more accessible for average citizens.

Coarse Thread pitch chart

Fine Thread pitch chart