. . . is what is mandated on ALL of the rat care websites. Like this one at: http://www.spoiledratten.com/housingcontent.html
where they say, ". . . get the biggest cage you can for your rats so they have plenty of space to move around and to play. Allow at least two cubic feet per rat and take into account individual levels. A tall cage with only the one level still isn't much space."

http://www.paw-talk.net/forums/f16/rat-cage-size-51490.html
"Rats need a minimim of 2 cubic feet EACH to be happy."

Here are some urls to cage size calculators:

http://www.rattycorner.com/odds/calc.shtml
"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."

http://www.ratclub.org/cgi-bin/cage3.cgi
"The guideline for this cage calculator is 2 cubic feet per rat. . . . however the bigger the cage the better."

I almost went to jail 16 months ago, because my rats were a little more crowded than two cubic feet per rat. That is what I mean about learning this the hard way. My county has a very mean jailer, so I am terrified of going to jail.

Snakey Lakey