Sockets: a guide to metric and royal measurements

Date:22 August 2017 Tags:, , ,

A standard socket-wrench set comes with two types of sockets, sized in millimetres and inches. A few will inevitably go missing. Luckily, some sockets match sizes in both measurement systems.

Metric: This is the system used by the most countries across the world. It is based on an internationally agreed decimal system of measurement.

Royal: Largely used in the USA, Myanmar and Libera, this system is also known as the system of imperial units. The royal measurement system has in recent years become synonymous to screen resolution.

Here are the ones to know, along with how close they are – the further apart, the higher the risk of damaging a fastener.

A sizing guide to sockets:


SAE size (in.)
Metric size (mm)
Difference (1/1000 in.)
5/32 4 1
29/32 23 1
5/16 8 2
3/4 19 2
19/32 15 3
7/16 11 4
15/32 12 4