Jean sizes are based on your waist measurement in inches. A soft tape measure, like tailors use, is most accurate. If you don't have one, use a length of twine, then measure that with the tape measure you've got in the garage.
Determine how high on your waist or hip you like to wear jeans; measure at this point. Keep the tape comfortably loose and parallel to the floor while you measure the full circumference of your torso. Make sure to measure your body, not over your clothes.
Grab jeans that already fit you well and lay them on a flat surface. Measure from the crotch seam down to the ankle hem.
Before making your final selection, check the product page to see if our Fit Specialists have any additional sizing tips.