Swimline 18' Round Winter Swimming Pool Cover + 4'x4' Winterizing Air Pillow Pillow Size: 4'x4' Pillow is used to push your pool cover up off the water's surface so the water won't freeze to your pool cover Pillow is used to keep water off the top of your pool cover Pool Size: Fits 18' Round Pools U.V. Inhibitors Protect the Cover From Sun Damage