It's best to stand barefoot in your underwear for measuring. For best results take your measurements a few times or get someone else to measure you. Take care not to hold the measuring tape too loosely or to pull it too taut.
Measure around the base of your neck where your collar would normally sit.
Measure around your chest at its broadest and fullest part at nipple height, avoiding placing the tape too high under your arms.
Measure around your natural waistline, at the point your waistband or belt usually sits.
Measure from the top of your inside leg, at the crotch, to the floor.