Cubic Inches to US Gallons (Liquid) conversion calculator
Cubic Centimeters to US Gallons (Liquid) conversion calculator
Those two might help

There are also converters to gallons dry, but I'm not sure what that really means..
I realize that a 12x12x12 is quite small for any frog, and I think I will just keep it as an emergency tank, because you can never know when you will need one.

I really like frogs, but at the time right now I couldn't get another frog anyways, thats something for after college
PS. Any stories or advice for taking a frog to college?? :P