This simple but effective item is the heart and soul of keeping your purified water contained. This 55mm crown or snap-on red cap is the industry standard cap for 3 and 5 gallon water bottles. Its standard intergrated 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