Hi mrfishy - you can probably find a tank volume calculator online, somewhere. I've seen them, but can't remember where. Mine is just a standard 10 gallon tank, which I used to leave me space for my ATO reservoir, to keep it all out of sight. Keep in mind that the sump/fuge won't be filled to the top, so you won't get the total volume that the sump could hold - a lot will depend on your baffles, plus you need extra volume to account for the water that will drain back into the sump when the return pump is shut off, or if you lose power.