[updated 2008-07-22]
This link contains the calculator (volume, filtration and heating requirements) and popular tank sizes
http://www.redearslider.com/calculator.html
For future reference, you can multiply Length (in.), Width (in.), Height and divide by 231 to obtain gallons of water.
Here are alternative links:
Aquarium Calculator — Unit Conversion Tables
http://www.cichlid-forum.com/articles/calculator.php
Tank Water Volume Calculator
http://www.garf.org/calculators/TankVol ... ulator.asp