Westminster Belt

To find the correct size, look at a belt you wear and measure from the leather starting point at the buckle end to the hole that you use. 

Alternatively you can measure your waist (do not go by trouser size, this can vary with brands) and add an inch. For example if you have a waist measurement of

34" you should buy a small/medium.

Measurements are taken from where the buckle joins the leather from to the first and last hole, your waist size should be near the middle.