Waist
Lay your pants flat on a hard surface. Measure straight across the waistband from edge to edge. Double that number.
Fold the pants in half first so the front and back sit evenly - this keeps the waistband from twisting as you measure.
Front rise
With the pants face up, place your tape at the crotch seam - where the fly ends and the two inseams meet - and run it straight up the center front to the top of the waistband.
This controls where the pants sit and how much room you have through the front. Most pants measure 9–12 inches here.
Back rise
Flip the pants over. Starting from the same crotch seam point, run your tape straight up the center back to the top of the waistband.
Back rise is almost always 2–4 inches longer than front rise. This is what determines seat comfort and whether pants pull down when you sit.
Seat
With the pants flat, find the widest point - usually about 7–9 inches below the waistband where the back pockets sit. Measure across. Double it.
Don't measure at the thigh by mistake. You want the broadest part of the hip, not where the legs begin to narrow.
Thigh
Measure across one leg at the very top, right below where the two inseams meet. Double it.
Keep the fabric flat and smooth. You're measuring one layer of a single leg, so doubling gives the full circumference.
Inseam
Run your tape from the crotch seam - where the two inseams meet at the bottom of the fly - straight down the inner leg to the hem.
This is a single straight length. No doubling. For raw or unsanforized denim, account for shrinkage if you haven't washed it yet.
Outseam
Measure from the very top of the waistband down the outside edge of one leg all the way to the hem. This runs the full length of the outer seam.
Outseam is always longer than inseam. The difference between them roughly equals your rise - cross-referencing the two lets a tailor verify your numbers instantly.
Knee
Measure across one leg at the midpoint between the crotch seam and the hem - roughly where your knee lands when wearing. Double it.
If the pants have a crease, seam, or reinforced knee area, use that as your guide. Otherwise just eyeball the midpoint.
Leg opening
Lay the hem flat and measure straight across the opening of one leg from edge to edge. Double it.
This number is what separates a slim fit from a wide leg. Don't stretch the hem when measuring - let the fabric lie natural.