it depends on what you want the substrate to be really. You have to be certain that you'll either be using burrowing dirt or one of the other substrates that make a layer of only up to 3". If you are going to go with the burrowing dirt substrate, then you'll want to have basically about 18-20 inches of height that will not have any substrate, and then acount for another foot or so for dirt (depending on how much you intend to use), which will be basically all substrate dam when you build the cage. If you go with sand, birdseed, or any type of paper product for substrate, you'll only want the cage to be about 18-20" in total. Also, you will need to decide if you are going to use external or internal lighting, meaning are the lights going to be mounted inside the cage, or set on a screened off section of the top of the cage? If you go for screened off section on top of the cage and you're going to have a low substrate (sand, etc.), then you could get away with only a 12" tall cage, though it would be hard to get into for cleaning purposes and the viewing surface would be absolutely tiny. If I were you, I'd go with 20" tall, with internal lighting, and then just add how ever many extra inches to the bottom of that if you're going to be using dirt substrate.