The AC Safe fan filter is custom-sized to fit almost all 20-inch square box fans. Just clip it to the back of your fan. It can be trimmed for a custom fit. This unique product is made of a special blend of polyester synthetic media that is designed to be washable and air dried, making it suitable for multiple uses. Use it to prevent dust from blowing throughout the room and keep fan blades clean. You’ll wonder how you got along without it!