The best way to determine the size belt you need is to measure your current belt from the fold near the buckle to the hole that you use. This will be the size you need.
Do not order by your pant size or waist size. You always need to go at least  one size above your pant or waist size.

Further detail:
When buying a belt, it is important to select the correct size based upon an accurate measurement. Your belt size is not the same as your pant or waist size; because your belt goes around you and your pants, your tucked in shirt, and your underwear, your belt size is usually 1 or 2 inches larger than you pant or waist size. The leading manufacturers of quality belts share the same standard for measuring and labeling belt sizes. This measure is based upon the length of the belt strap only, and does not include the buckle.
Most belts are normally measured from where the belt leather folds at the buckle end to the middle hole at the tip end and are usually stamped on the back of the belt with the size example 32, 34, 36 etc. Many women's belts are marked with a letter such as XS, S, M, L, or XL in general, those sizes would be: Womens Size Scale XS = 24" - 26" S = 28" - 30" M = 32" - 34" L= 36" - 38" XL = 38" - 40".

For most belts you would want to choose your belt approximately 2" larger than your waist measurement, of course we would suggest that you measure your waist at the level where you would wear your belt. The best general rule of thumb is that you should buy your belt 2" larger than your waist - pants size, but you should never hesitate to ask questions when in doubt.