Take a loose tape measure and wrap it around your wrist. You could also use a string and just measure the length you need based on how it fits. Keep in mind that if you plan to wear several pieces at a time you may want to pick a size up for the ones that will be worn higher on your arm. Due to variations in bead size and shape there may be a .5” variance in size.
Typical Women's Sizes
Typical Men's Sizes