They are the 41 quart Steralites...I use them for my birthing quarters...Gives them more room to move and for the little ones to grow in...It takes more space but I've been lucky with a bunch of litters and 9 are down there right now...Thus, going and tossing them out and getting new bottoms gets pretty expensive...Once they are really chewed I will, but just trying to save some $$ too...