I've finally written up a script to calculate the total water volume for any square or rectangular container for us all to use, unless you prefer doing the math yourself there is a link to the Tank Volume Calculator on the Home page under Calculators. (yes I intend on writing more)
It's quite 'plain jane' at the moment but should be fully functional for calculating to US gallons, I'll add more bells and whistles, and clean up the page over the next few days.
As always your suggestions and feedback will be welcomed.