The most accurate way to find your belt size is to take a measuring tape and a belt you currently own. Measure (in inches) from under 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.
If you don't have a belt, you can measure your waist where you wear the belt and add 2" to this value. For example, if you measure your waist to be 36", then you need a size 38 belt.