# Easy SAE to Metric socket size conversion

There are approximately 25.6 millimeters per inch.

What’s that? You say there are only 25.4 millimeters per inch?

You’d be right in saying that 25.4 is a closer approximation, but 25.6 is more useful, because it leads to this rule of thumb:

There are 256 millimeters in 10 inches.

Since 256 is a power of 2, this makes it really easy to convert between metric socket sizes (which are in millimeters) to SAE socket sizes (which are in power-of-2 fractions of an inch).

Consider a 1⁄2” battery terminal. If there are 256mm/10in, then there are 128mm/in, or 12.8mm per inch. So a 13mm socket will be ~.2mm larger than the bolt head.

I find going the other way a little trickier, but not too bad. For the 13mm socket, I remember that 1mm is 10*1⁄256”, so 13mm is 130⁄256”. The closest power of two to 130 is 128, and 128⁄256” is 1⁄2”. So a 13mm socket is just 2⁄256” bigger than a 1⁄2”.

Why is this useful? Sometimes it useful to use a socket size that’s close rather than an exact match. A battery terminal is usually a 1⁄2” socket, but what if you’re on the side of the road and you only have metric sockets on hand? Grab a 13mm and it will be a close enough fit. Or what if your 13mm bolt head is rusted? Chances are it’s not a 13mm head any more, and a 1⁄2” socket will be a tighter fit.

Here’s a full conversion table for common socket types:

SAE Metric   Metric SAE
1⁄8 3.2mm   8mm 5⁄16
3⁄16 4.8mm   9mm 11⁄32 - 3⁄8
1⁄4 6.4mm   10mm 3⁄8 - 13⁄32
5⁄16 8mm   11mm 13⁄32 - 7⁄16
3⁄8 9.6mm   12mm 15⁄32
7⁄16 11.2mm   13mm 1⁄2 - 17⁄32
1⁄2 12.8mm   14mm 17⁄32 - 9⁄16
9⁄16 14.4mm   15mm 9⁄16 - 19⁄32
5⁄8 16mm   16mm 5⁄8
11⁄16 17.6mm   17mm 21⁄32 - 11⁄16
3⁄4 19.2mm   18mm 11⁄16 - 23⁄32
13⁄16 20.8mm   19mm 23⁄32 - 3⁄4
7⁄8 22.4mm   21mm 13⁄16 - 27⁄32
15⁄16 24mm   23mm 7⁄8 - 15⁄16
1 25.6mm   25mm 15⁄16 - 1

Updated: