Measure your wrist
Cut a piece string or paper to a length of around 20 - 25cm. 	
Wrap the string around your wrist, just above your wrist bone. Pull until it sits comfortably - not too tight, but not so loose that it could slip over your hand. Personal preference will also come into play here, depending whether you want a closer fit or a loose fit to be slung low over your hand. 	
Use a pen to mark the overlapping parts of the string or paper.	
Lay the string/paper out flat against a ruler or tape measure, and measure between the end of the string and the point you have marked. 	
This measurement provides your wrist circumference. 
Please note, if you're shopping for a bangle which is designed to slip on and doesn't feature a hinge to open and close, you'll need to allow a little extra space for your bangle to slip over your hand.