When replacing O-rings for carboy fermenters ensure you purchase the correct O-ring for your home brew equipment.  O-rings come in different sizes to fit various carboys:

AMPI - 10mm thick with a 245mm diameter

Atlas - 7mm thick with a 270mm diameter

BMW/Rheem - 10mm thick with a 190mm diameter

Davis - 8mm thick with a 245mm diameter

VB - 8 mm thick with a 220mm diameter

* Make sure you measure the O-ring you want to replace so your new O-ring is a correct fit!