This simple but effective item is the heart and soul of keeping your purified water contained. This 55mm Crown or snap on light Blue cap is the industry standard cap for 3 and 5 gallon water bottles. Its standard integrated cup seals both the interior and exterior neck of the bottle. Release tab on the outer edge makes removing the cap a breeze. The cap fits both glass and plastic 3 and 5 gallon bottles with a "Crown" or "snap on" top. For reference, outside dimension for the bottle opening would be 2.165 inches. If you currently have a snap cap for the bottle the outside dimension of that cap is about 2.75 inches.