Measure a comfortable pin belt from the buckle fold to the hole you use most, excluding the metal tip.
Treat trouser waist plus one to two inches as a starting guide, then check the seller’s own chart.
Measure around the waistband with your intended trousers on, keeping the tape level without compressing your body.
Between sizes, compare usable holes and tail length rather than assuming the longer belt will always fit better.
Check strap width against the trouser loops and test buckle comfort while sitting, as well as checking length.
Read the full guide to measuring an existing belt and comparing the seller’s size chart.