This Universal Rubber Stopper is made with a taper and of a different material than the rubber gum stopper so that it fits multiple sizes of carboys and does not pop out when wet. It also has a lip around the top so it won’t push into the carboy either. Small (sometimes called “regular”) fits most standard carboys. This is the equivalent of #6 – 7
Diameter at Top: 1.5625″
Diameter at Bottom: 1.125″