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 for two or more rats, 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, ropes 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 in. Rats kept in a large and enriched environment with both ratty and human interaction will develop more character and make better pets.