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:
|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|
|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|
|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|