To measure seat, pant should be flat, zipper fastened ; measure straight at the base of fly tack.
True rise measurement
To measure true rise - first you measure outside leg seam from the waistband to the bottom of leg seam.
Second, using the same leg, you measure the inside leg seam from the crotch point to the bottom of inside leg seam. The difference between the outside seam and inside seam is your true rise.
To measure the inside leg seam, you must measure from the top of inside seam to the bottomof inside seam.
To measure knee, you must measure from the crotch point (top of inside seam) 13" down; then measure a straight line across the leg, making sure the leg is very flat.
To measure the thigh, you must measure a straight line from back to front from crotch point (top of inside seam), making sure the leg is flat down.