Measure yourself along the high hip at the bottom of your belt loops on your jeans/pants/chaps/hunting gear.
Provide enough inches to buckle your belt at the middle hole of your gun belt.
Measure yourself over your usual shooting attire. Generally speaking, you'll wear your gun belt on your high hip, with the top of the belt resting near the bottom of the belt loops on your jeans/pants/chaps/hunting gear.
Gun belt sizes are usually 4-6 inches larger than your regular blue jean waist size.