1. Place your thumb across your hand
2. Wrap string or ribbon around the widest part of your hand (could be across from your lower thumb knuckle or across your hand - see pictures)
3. Measure string or ribbon
4. Divide the measurement by pi (3.14) to get the diameter, which will be the diameter of the bangle you need
Please note - in theory, you could size down if your hands are flexible and you're able to slide the bangle through your hand. Otherwise, stick with the widest part of your hand as the measurement.