We have a very big AC unit (over 38x34 inches), and the 36 inch cover was large enough to completely cover the fan opening on top of the unit, with the cover's corners overlapping the AC unit sides enough to allow secure fastening via the attached elastic cords and S hooks. 30 seconds to install. Not the thickest material, but seems sturdy enough for the intended purpose of keeping leaves and snow out of the AC unit innards. We'll have to see how this cover holds up to sun exposure and Michigan winter weather.