Your natural waist is the narrowest part of your torso, typically 1–2 inches above your belly button. It's NOT where your jeans usually sit (that's your hip line).
Stand naturally — don't suck in your stomach. Wrap a soft measuring tape around your waist, parallel to the floor. The tape should be snug but not compressing your skin.
Take the measurement at the end of a normal exhale. Repeat 2–3 times for accuracy.
Common mistakes: measuring over thick clothing, pulling the tape too tight, or measuring at the hip instead of the waist. All three can throw off your size by 1–2 numbers.
If you don't have a soft tape, use a piece of string and then measure the string against a rigid ruler. Works just as well.