The Number of Rats You Will Have
If you have only one or two rats, a small enclosure such as a 20 gallon aquarium will do, or a medium or large-sized wire cage with a plastic bottom. If you have more than two rats, you should make sure you give them enough room to move around so that they aren't always crammed together (though you will find they often do this on their own!). Cages with a large number of rats can be relatively small, but aren't usually a good idea. They tend to smell and need cleaning often. It is better to use larger cages for two or three rats at a time. The general rule is there should be two cubic feet of space per rat.
The Sex of the Rats You Will Have
Male rats scent-mark, so their cages need to be cleaned more often than female's cages do. In this case, an aquarium isn't advised, as the odors tend to sink to the bottom and don't escape the glass walls. Also, the smell of the urine and feces can linger inside a glass aquarium, making it rather unbearable for your pets. A wire cage is almost always preferred. However, you have to make sure that the rats cannot fit through the bars.
The Amount of Space You Have
If you don't have space for a wide or long cage, simply get a tall cage with two or three levels. Even an old bird cage, provided that the wires aren't too far apart, can satisfy your rat's housing needs.
What You Are Going to Put in the Cage
If you want your rat to be healthy and happy, you need to give him some toys to play with. Running wheels are always good for rats, though some rats tend not to use them, but you have to make sure you are going to be able to fit it into the cage without obstructing anything else. Plastic pipes can be great fun for your young rat because they mimic the tunneling behaviour that is natural to rodents. You also need to make sure that your water bottle is compatible with the type of cage you have chosen. If you have a glass or plastic cage, you will need a suction cup holder or a special aquarium water bottle metal holder to fix the bottle to the side of the cage. If you have a wire cage, make sure you have a wire or plastic hanger so that the bottle will stay upright and not drip.
