If you only have a ruler or something else you don’t want to stand on, you can trace your feet and use that to measure the length. The Michigan Foot Doctors YouTube channel has an in-depth guide to proper tracing and measuring:
Whichever technique you use, make sure you actually put weight on each foot as it’s being measured so you get as accurate a number as possible. Once you’re done, save both measurements in a note on your phone, Google Drive, Dropbox, or password manager. (A Post-it on the fridge works, too.)
Using your measurements really takes the guesswork out of finding your size. Let’s say your left foot is 26.3 centimeters and your right is 26.5. For shoes that need a snug fit and/or will likely stretch out, look for the size that corresponds to 26.5 or 27 centimeters. If you want a roomier fit—such as for boots you’ll wear with thick socks—go for 27.5 or even 28.
That’s it! You may not get the perfect size every single time, but you’ll get a lot closer than you would with traditional number sizes. “Size 9″ can mean a million different things, but you can always trust the centimeter.