With a tape measure:
Wrap the tape measure around the wrist on which you plan to wear your ShopApes bracelet. Make sure the tape measure is close to the hand, below the wrist bone where you would normally wear your bracelet. Pull the tape measure tightly, but without cutting off your blood circulation, around your wrist. Write down the size which is indicated where the tape measure meets the 0. The size can either be in inches or cm.