"How many rats will fit in the cage I have?"
This is the age-old question! Personally, my guideline is about 2 cubic feet per rat (0.6 cubic meters). Below is a link to a Java-based applet that will do the math for you, or you can figure it out yourself with some basic math and geometry using the formula that follows the link.
Keep in mind that this calculator is only intended to be used as a guideline. It does not take into account the levels and balconies in a particular cage, nor does it consider the personalities of individual rats. Some rats need more space, some need less. Every rat is different. The formula is based on adult males, however, as females are generally more active than males, they may require the same amount of space, despite being smaller in stature.
Notes: Java MUST be enabled on your browser in order for this program to work. It may not work on some browsers at all, which is why I have also posted the formula below.
EXAMPLE Martin's R-695
30" x 18" x 36" =19,440
Divide by 3456