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 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.
The RSPCA (UK) have guidelines as to the sizes of cage required for various animals. For rats the suggestion is:
Rats (L x W x H) Sociable species (Like to live in pairs/trios/groups)
Cage approx size - 6sq ft (3ft x 2ft on 1 level) OR (2ft 6'' x 1ft 6'' x 3-4ft high on 2 levels).
PLEASE NOTE that these sizes are GUIDELINES and there will always be an element of flexibility providing good welfare standards can be maintained.
Although this states that rats like to live in groups, the sizing gives no indication of the number of rats such a cage could reasonably house. It would be interesting if pet shops were obliged to label cages according to these guidelines, and if such guidelines were applied to rodent farms breeding for pet shops and reptile food, or indeed within RSPCA rescue centres.