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 will not fit the screw on type of bottle, BPA free, made in USA.