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