Measure around the middle of your neck, over your adam’s apple, following the natural curve.
Measure around the widest part of your chest, underneath your armpits - relaxing your arms at your sides. Make sure you keep the measuring tape level, all the way around.
Measure around your upper waist, at the smallest part of your torso. This is usually right at the base of your rib cage.
Measure around your belly at the widest point. This is usually right below the belly button. Don’t flex, and don’t suck in your gut.