#10 Drilled Rubber Stopper (DRS)
The #10 Drilled Rubber Stopper (DRS) is a pure white gum laboratory stopper. The top diameter is 1 15/16", the bottom diameter is 1 5/8", and it has a 3/8" hole drilled in the center, the perfect size for an airlock.
This is size of stopper you'll want to use with Better Bottles.
This stopper is also available as a #10 solid rubber stopper (srs).