A belt labelled with a size can fit a waist several inches away from that number. Nothing about belt sizing is standardised, including where the measurement is taken from.
There are three plausible measuring points
A belt can be measured from the buckle post to the middle hole, from the end of the buckle to the middle hole, or across the total length including buckle and tip.
Each convention is used somewhere, and the difference between them can run to several inches on the same physical belt.
Because no label states which method was used, a buyer comparing two belts by their stated size is often comparing different things entirely.
The middle hole is a moving reference
Most belts have five holes, and the intended fit is at the centre one, leaving two holes of adjustment in each direction.
Some makers use seven holes, and some space holes at wider intervals, so the total adjustment range differs even when the centre point matches.
A belt at the end of its adjustment sits at an angle through the loops and wears unevenly, which is why the centre hole is the reference rather than the fit range.
Trouser waist is not the same as body waist
A belt sits over trousers, so its effective length must cover the trouser waistband circumference rather than the wearer's measured waist.
Rise complicates this further. A low-rise trouser sits at a wider point on most bodies than a high-rise one, so the same person needs different belt lengths.
Advice to buy a belt two inches above trouser size works only where the trouser is labelled with a real measurement, which many are not.
Regional systems add another layer
Some markets label belts in centimetres, some in inches, and some use small through extra-large mapped onto a range that varies by brand.
Conversions between systems are usually done arithmetically without accounting for the measuring convention, so an error survives translation.
The result is that a belt bought across markets in a familiar size can arrive noticeably long or short with no defect involved.
What makes a belt hold up
Full-grain leather cut in one piece flexes without delaminating. Bonded leather made from ground fibre and adhesive cracks along the fold within a season or two.
Stitched edges outlast glued ones, and a buckle attached with screws or a removable pin allows both shortening and buckle replacement later.
Since a well-made belt is shortenable at the tip, buying slightly long is the safer error, which is a practical answer to a sizing system that will not standardise.