Bucks Club belts are designed to match your normal pant size. For instance, if you wear size 34 pants, a size 34 Bucks Club belt is the one for you.
If you do not have your go-to Levi's at hand to look at your normal pant size (and for some reason you do have a measuring tape close by), take a look at these diagrams to help you figure out which is the right size for you.
Remember that a part of the end of the belt (2-3 in/10-15 cm) is supposed to overlap while wearing.
You can also compare these measurements to another well-fitting belt. We would say a 'favorite' belt, but that's about to be your Bucks Club belt, of course.