For the waist, lay your jeans flat and buttoned. Pull the front waistband upwards so that the top edge is flush with the back waistband. Measure the waistband from end to end and multiply this number by two.
For the thigh, measure straight across the leg 1” down from the crotch.
Knee measurements are taken exactly 14” down from the crotch seam.
For the leg opening, measure across the bottom of the leg at the opening.