High Output Replacement DiscsThese lightweight, high speed replacement discs will get your tired mist maker fogging again! ALL House of Hydro mist makers use 20mm discs. If you are buying discs for a generic mist maker, it may require a 16mm disc. The easiest way to measure your disc size is by using an American Penny. The 20mm discs are slightly larger, and the 16mm discs are slightly smaller than a penny. You must remove the disc to measure it. DO NOT use the black rubber rings