Take a measuring tape & measure all around your waist (where you wear your pants). This is your body waist measurement.

Lay flat anyone of your well fitting trousers and measure from the bottom of the crotch (where the inseams meet), down the inside of the leg to the bottom of the leg opening.

Lay the pants down on a flat surface, spread the pants out flat, with the waistband buttoned at the front and the zip closed. The front of the waistband should lay directly on top of the back of the waistband. Measure across the top of the waistband from one side of the pant to the other double the number.