48mm cap - Inner Diameter: 48mm, Outer Diameter: 50mm | 53mm cap - Inner Diameter: 53mm, Outer Diameter: 55mm | Caps include a white liner to assure a tight seal between bottle & cap. Features ribbed sides for ease of grip when removing cap. | Replacement Caps are for use on 3-gallon and 5-gallon water bottle jugs. Please measure jug for proper cap size selection. | 3 caps included. For proper fit, measure the OUTSIDE diameter at the top of the jug's neck opening. The size 48mm cap will work for jugs with an 48mm OUTER diameter neck opening, and the size 53mm cap will work for jugs with a 53mm OUTER diameter neck opening. These are also THREADED caps, and will only work with THREADED jugs.