What are calculations for gallons of water square tank?

1 US Gallon = 231 Cubic Inches So if you find out the cubic inches you can convert it to gallons. Example: If your tank is 3 ft wide by 2 ft deep and 2 ft high.

36 inches x 24 inches x 24 inches = 20736 cubic inches / 231 per gallon = 90 gallons