How many ratties will be happy in your cage?
The cage calculator gives a guideline to how many rats your cage can hold. It's not set in stone. As a default it allows 2 cubic feet per rat, which gives results that generally coincide with the number of rats which the experienced rat keeper is happy to house in a cage. It doesn't take account of the number of shelves your cage has, as you can also add hammocks and other toys to use your cage to it's full potential.
* Remember, you owe it to your pets to give them a comfortable and happy home, not just the minimum they can survive on.