Easy SAE to Metric socket size conversion

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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:

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