Spanner and socket sizes

Metric and imperial spanner sizes across the flats, with the nearest size in the other system and whether it can safely be used.

Four pairs are interchangeable. Several will destroy a bolt.

Spanner sizes are measured across the flats, so comparing systems is just arithmetic — and the arithmetic decides whether reaching for the wrong drawer is fine or expensive.

These are close enough to swap:

These are not:

Which way the damage runs

The failure is asymmetric and worth getting straight. A spanner larger than the head is the dangerous one: it sits on the corners rather than the flats, and under load the corners deform. Once a head is rounded it needs an extractor or a cutting disc.

A spanner smaller than the head simply does not go on. That is the harmless failure — annoying, and it tells you immediately that you have the wrong tool, which the dangerous one does not.

The classic case is 13 mm on a 1/2 inch bolt. The head is 12.70 mm and the socket is 13, so it goes on, feels almost right, and rounds the fastener on the first hard pull.

Metric sizes

Imperial sizes

Metric threads — the tapping drill and clearance hole for the bolts these turn. Unit converters