Originally Posted by idroba
Take a gallon jug, and fill it with water. Dump into the tank. Repeat until full. Count the numbers of jugs. Works with all tanks.
Depending on the configuration of the tank, you can use the following formula to determin the tank capacity for water.
Length X Width X Heighth in inches, divided by 231, equals gallons.
Of course you must subtract a little because of the tank thickness.
AS an example, what is the capacity of a cubic foot? 12" X 12" X 12" equals 1728. Dividing that by 231 gives you almost 7.5 gallons.