This cast cover is completely watertight, keeping your bandages, prosthetics, and casts dry. It is the only leg and arm cast protection in the world with the patented vacuum seal to keep your bandage or cast dry. It is so completely and totally waterproof, between the waterproof fabric and the vacuum seal, that you can even dive off a diving board, ride the ocean waves,or scuba dive and still have your cast stay completely dry. Arm and Leg Cast Covers are designed to fit snugly, especially the opening at the end of the cast cover where the arm and leg cast covers must remain sealed. To create a good vacuum seal the opening for your specific cast cover needs to be smaller than your limb. LEG : Measure from the back of the heel (from the floor) to the top end of the cast or prosthetic. If the cast ends close to the knee you will need to measure up to the mid thigh. Don't worry if the waterproof cast cover is too long. The excess material of the cast sock will flatten down when the air is pumped out. Measure on the skin above the cast, not on the cast itself. Make sure that the cast cover is at least 2 inches longer than the cast. 2. Measure the circumference of the leg just above the cast or bandage (If the end of the cast is very close to the knee, measure mid thigh.) Notes: Do not leave the cast cover on for extended periods of time. We recommend removing it after every 45 minutes of use. Do not use in vascular compromised patients. Steady yourself in wet slippery areas. Always follow your doctor's advice.