I never work on waist size . . . I prefer to use 'fixing length', which is always bigger than waist or pants size. Some folk wear belts around their waists, and others wear them around the hips, and the difference in length can be surprising!
I always send my customers a picture (see below) which shows how to measure the fixing length from an existing belt, and if they can't get that right, then I charge them for the remake . . . very politely, of course!