Each bag of PrettyLitter is designed for a medium or "average" size litter box around 16 inches long and 12 inches wide. PrettyLitter requires a minimum depth of 2 inches for a litter box being used by one cat. One bag will accomplish that. If you use a larger litter box you may want to consider buying an extra bag in order to provide proper depth. If your cat is older or on the larger side, we recommend you get your litter box up to at least 3" of depth for optimum absorption. In this case, we'd recommend buying an extra bag of PrettyLitter and use just half of that bag along with the first bag. Additionally, if you have a cat that loves to dig, that extra bag will get you to at least 3 inches of litter lining the surface of the tray. If you have a multi-cat household, we recommend a minimum of one bag of PrettyLitter per cat. For example, if two cats are using one litter box, we suggest two bags. If each cat has their own litter box, but one is used more than the other, we suggest two bags per box. There are many different sizes of litter boxes, and every household's litter box usage is different, so if you ever need more or less litter, we're happy to help anytime. Feel free to contact our CS team at meow@prettylitter.com or 800-838-3381.