Size chart for your arms or ankles - Small 9" - 15" diameter - Medium 13" - 19" diameter - Large 17" - 23" diameter Unlike reflective products, NITE BEAMSTM provide you with HIGH and INSTANT VISIBILITY for a SAFER workout from the front, from the side and from behind. Our LED products provide approx. 1400 feet of visibility (about 1/4 mile) allowing time for a driver to SEE and REACT to you. By simply pressing the on/off button on the battery pack, each arm/leg band can effortlessly be adjusted to run in one of three modes (on-blink-off). Secure the arm/leg band on your arm or leg using the high quality velcro, rotate the band on your arm or leg until the battery pack is in the back (tricep or calf) and you are immediately visible from three sides (front, side and back). Remember SAFETY FIRST, SAFETY ALWAYS!!! Your family wants you SAFE and so does NITE BEAMSTM. That is why we manufacture and sell the WORLD"s best arm/leg/wrist bands.