Dimensions of 53mm size cap: Inner Diameter: 53mm (2.09 inches), Outer Diameter: 55mm (2.17 inches). 53mm cap for use on models such as: American Maid. Please note that these caps aren't universal, as there are many other cap sizes available. Each water bottle brand has it's own neck design, and therefore requires a specific cap size based on the width of the neck and closure type (screw-on or snap-on). For example, the 53mm sized screw-on cap fits the 1, 2, 3, and 5 gallon American Maid brand bottles. And the 48mm sized screw-on cap generally fits on the 1, 2, 3, and 5 gallon Bluewave, New Wave Enviro, PureAqua, H8O, and LavoHome brand bottles. These do not fit on bottles that require Snap-On caps (where the bottle's lip is rounded and has no thread). Brands that typically require Snap-On caps: Nestle, Primo, Zephyrhills, Crystal Springs, Poland Spring, Arrowhead, Deer Park, Ozarka, Pure Life, Brio, Polar Arctic, and Clack brand bottles. These do not fit on glass bottles.