Every Levi’s® piece has a handy sizing guide to help you find your size. For example, a measurement of 85cm in your waist, 105cm in your seat and 58cm in your thigh is considered a size 32, or a size M.
Here's a guide on how to measure:
Waist for Bottoms
Place the tape horizontally around the smallest part of the waist.
Seat
Place the tape 20cm below the waist. Make sure not to move the tape up and down.
Thigh
Place the tape horizontally around the thigh, 5cm under the crotch.
Inside Leg
Place the tape on the inside of the leg and measure from the crotch to the ankle.