The simplest way to find your belt size is to determine your pant size and add 2 inches. For example, if your pants are size 32, you will need a size 34 belt.
Alternatively, take a measuring tape and a belt you currently own. Measure (in inches) from the tip of the buckle to the hole you use to close the belt. That value will be the size you need. For example, you measure your belt from the buckle to the hole you use and find it is 36''. This means you need a size 36 belt. If you prefer a little extra hang, add two inches to this value.