If you carry an inside the waist band (IWB) holster you must measure with your holster in place.
How to size your belt using a belt you already have.
Measure a belt you have now from the end of the buckle to the hole you use most often (to the nearest inch). This measurement should be about two inches larger than your waist or pant size.
If you are measuring an old belt that has already contoured (has that curve in the back) to your body – I recommend you use the “cloth tape measure” method below to obtain the correct measurement. That curve is difficult to get out, so you’re guessing at the correct size.
How to size your belt when ordering using a cloth measuring tape.
Put a cloth tape measure through the belt loops on your pants and pull it a little snug.
If your measurements end up to be in between inch marks you should round it up to the next larger inch mark.