Socket Size Conversion Chart

Metric to standard socket size chart

The gap column is what matters. Under 0.004 inch the socket seats properly. Between 0.004 and 0.008 inch it will go on but only use it at low torque and only on a fastener in good condition. Over that, find the right socket.

The two conversions worth remembering are 19mm to 3/4 inch, which is 0.002 inch apart, and 27mm to 1 1/16 inch at 0.0005 inch. 17mm to 11/16 inch is the one people reach for and it is 0.018 inch loose, which is how corners get rounded.

Standard wrench sizes

A standard SAE spanner set runs 1/4 through 1 inch in sixteenths, with the half sizes filled in from 3/8 upward. A metric set runs 6mm to 24mm. Between them they cover almost everything on a machine, but you cannot substitute freely, as the table above shows.

Wrench size is the distance across the flats of the fastener, not the thread size. A 1/2 inch bolt takes a 3/4 inch spanner. An M8 bolt takes a 13mm.

mm to inches chart

One inch is 25.4mm exactly, so millimetres to inches is a division by 25.4. The conversions below are the ones that come up on a bench often enough to be worth knowing without a calculator.