To determine your cuff size, use a tape measure to find the circumference of the uppermost part of your arm or leg. For your leg, measure around the top of your thigh. For your arm, measure just above the biceps, beneath the armpit.
Choose the cuff which is 1-6" longer than your measurements.
Examples:
Your arm or leg has a circumference of 28" -> choose the 30" cuff.
Your arm or leg has a circumference of 19" -> choose the 24" cuff.
Your arm or leg has a circumference of 17" -> choose the 18" cuff.