A lot of people think your heel-to-toe measurement (the overall length of your foot) is all you need to determine what size shoe you should wear. But that is only part of the information you need. While the heel-to-toe measurement is important, the heel-to-ball joint measurement (your arch length) is a better guide to getting a proper fit.
Shoes are designed for the ball of your foot to be at the widest point of the shoe. This means sizing your feet from your arch length should give you a proper fit. You should make sure to measure both feet, yes both feet. This is because your left foot and your right foot are not always the same size and some people have long toes and some may have short ones. That is why you need both measurements.
As a general rule, if both measurements are within a half size of each other then start with the arch length size. For example, if the arch size is a 9 and the toe-to-heel is a 9 ½, then start with size 9. If it is a whole size apart, then start with the middle size. For example, if the arch size is a 9 and the toe-to-heel size is a 10, then start with a 9 ½.
As always, any sizing device is a starting point. Different shoes fit differently. So try them on and select the size that’s best for you. And remember to get your feet sized regularly. Feet do change size as we get older!