Pant sizes are based on your waist measurements in inches. To get the right size, grab a tape measure and follow the directions below. (One note: A soft tape measure, like tailors use, is most accurate.)
Determine how high or low on your waist or hip you like to wear your pants, and measure at this point. Keep the tape comfortably loose, and measure around the full circumference of your torso.
Grab a pair of pants that fits you well, and lay it on a flat surface with the leg out straight. Measure from the crotch seam down to the ankle hem.