Using flexible measuring tape, measure your wrist below the wrist bone. If you are between two sizes, select the smaller size for a tighter fit, or larger size for a looser fit. You can adjust bracelets about 1/4" smaller or larger.
If you don't have flexible measuring tape, you can use this paper version: DOWNLOAD PAPER MEASURING TAPE.
GENDER NEUTRAL SIZING:
XSMALL 
5 5⁄8 - 6 inches
SMALL
6 1/8 - 6 1⁄2 inches
MEDIUM
6 5⁄8 - 7 inches
LARGE
7 1⁄8 - 7 1⁄2 inches