the small critter keeper holds about .8 gals .. i tested this two ways ..
1. using the calculator .. don't use the measurements given for the dimensions from the paper .. measure it yourself .. from inside the container to inside .. the measurements on the box is for the widest point .. and u want to measure actual volume ..
2. using my 1g jug .. that i only use for aq water .. that jug is exactly 1g .. and i used that to fill my critter keeper .. and it only filled up .8 .. since i had about 2 in. of water leftover in my jug
so i think the calculator is pretty accurate .. i use 2 different one's .. to compare .. and i also do my own measuring .. since posted sizes have to be at the widest point .. and the plastic counts in that measurement .. so if the plastic adds up to .25 in .. it's included (which it shouldn't be when ur calculating volume) .. it's very misleading
the mini size critter keeper holds about .29g
again .. i measured from the inside of the plastic to get accurate volume .. not what it says on the container for dimensions