A belt bought in the same size as a pair of trousers frequently fastens at the last hole or falls short entirely. The mismatch is systematic and it comes from how belts are measured.
The stated size refers to the middle hole
Most belts carry five holes, and the size printed on the label is the distance from the buckle's prong to the centre one, not to the end of the leather.
That gives roughly two inches of adjustment in each direction, so the middle hole represents the intended fit rather than the maximum reach.
Total strap length is therefore several inches longer than the stated size, and confusing the two is why a belt measured flat never matches its label.
A belt sits somewhere the trouser waist does not
A trouser waist measurement is taken at the waistband. A belt passes through loops and around whatever the body measures at that height, plus the trouser fabric itself.
Fabric thickness, shirt tails, a tucked jumper and how the trousers sit all add circumference the trouser label never accounted for.
Low-rise trousers compound it further, since the belt then sits at the hips rather than the natural waist, where the body is usually wider.
Two inches up is a convention, not a rule
The common guidance of buying a belt two inches larger than trouser size works because it corrects for the fabric and the middle-hole convention at once.
It breaks down at the extremes of a size range and on trousers cut to sit unusually high or low, where the correction needed is larger or smaller.
Measuring an existing belt from the prong to the hole actually used gives an exact figure and removes the guesswork entirely.
Buckle style changes the effective length
A frame buckle with a prong uses one of a set of fixed holes. A plate buckle attaches over a hook and often offers no adjustment beyond punching new holes.
Reversible belts with rotating buckles add hardware depth at the front, which consumes length and shifts where the strap ends when fastened.
Ratchet belts abandon holes for a toothed track, giving adjustment in small increments and sidestepping the sizing question, at the cost of a bulkier mechanism.
Dress and casual belts are sized to different jobs
A dress belt is narrow, sits at the natural waist and is intended to fasten near the middle hole for a clean line under a jacket.
A casual belt is wider, sits lower and is often worn intentionally looser, with the tail length itself treated as part of the look.
Buying both in the same nominal size will produce two different fits, which is worth expecting rather than treating as an error by one of the makers.