A belt is measured from where the leather starts to the centre hole.
Due to the type of leather, this distance may vary by 12.5mm or so.
Don’t forget that belts go on top of clothes. So if you are a size 30″ you will need a to add a couple of inches to that.
Decide how you are going to wear your belt if you are going to wear the belt on your hips. You will need a much bigger belt than your waist size, so you should measure around the place where you will be wearing the belt.