You're looking for a trouser that never feels tight, yet sits securely at the waist without a belt. There should be minimal signs of stretch in the waistband - horizontal creases at the button or the top of the zips suggest you should consider a size up.
Measure around your waist, just above where you like your trousers to sit - ideally an inch below the belly button. Don't breathe in - we're not judging. Measure in centimeters and plug into the calculator below for your size.