One of things that almost every home winemaker uses is a rubber stopper. It's used to attach an airlock to a plastic fermenter or wine carboy. The rubber stopper is tapered in shape, has a whole in it for the airlock, and comes in various sizes. Knowing what size of rubber stopper to purchase can be of some issue. Whether you are using a gallon glass carboy or plastic fermenter makes a difference as to the size rubber stopper you need. I thought it would be nice to go over, "what size rubber stopper fits what", since we offer 25 different sizes to choose from. I should start of by saying that any bucket fermenter we offer takes a size #2 rubber stopper. This may not be the case with a fermenter purchased elsewhere. It depends on the size of whole the supplier decides to put into the plastic fermenter. Glass carboys come in an array of sizes and take a different sized rubber stopper as the size changes. There are 6.5 , 6 , 5 and 3 gallon sizes. The rubber stopper size for each is as follows: 6.5 Gal. Glass Carboys (Size #6.5) 6.0 Gal. Glass Carboys (Size #6.5) 5.0 Gal. Glass Carboys (Size #7) 3.0 Gal. Glass Carboys (Size #7) 7.0 Gal. Fermonster Large-Mouth (Size #10) 6.0 Gal. Fermonster Large-Mouth (Size #10) You may need to size a plastic wine carboy with a rubber stopper as these have become more popular over time. A plastic carboy takes a size #10.0 rubber stopper regardless of its size. Other sizes of rubber stopper you may need to know is are: Wine Bottle Opening (Size #2.0) Beer Bottle Opening (Size #2.0) Large-Mouth Gallon Glass Jugs, Quick Quarter Turn Lid (Size #8.0) Small-Mouth Gallon Glass Jugs, Thread-Down Lid (Size #6.0) Visit our website. It lists all of these rubber stoppers and more along with their dimensions. ----------------------------------- Ed Kraus is a 3rd generation home brewer/winemaker and has been an owner of E. C. Kraus since 1999. He has been helping individuals make better wine and beer for over 25 years.