Wanted to give you just a little info on this since it isn’t as obvious as it sounds. To get a good measurement the best thing you can do is take a fabric measuring tape and wrap it COMFORTABLY around your wrist at the thickest part. It helps to have a helping hand do this! Please keep in mind, if you pull the tape taught such that it digs whatsoever into your skin - this will be how the cuff fits. You may not notice how uncomfortable the measuring tape is - but you won’t love how the cuff fits! So yah, keep the tape comfortable - not dangling off you, but not digging in either. Also - I really do prefer the metric system! Millimeters are awesome!
