The general rule of thumb for determining your belt size is to take your pant size and add two inches to the value. For instance, if your pant size is 32, your ideal belt size is 34.
A more precise method is to measure your waist before purchasing a new belt. New leather belts are firm initially and increasingly mold to your body with use. The optimal belt size fits comfortably on the center hole (third from the top), with additional holes in case your waist size changes.