Calculating the volume of water in a square tank is a useful life skill. This can be used to determine the amount of conditioners and chemicals to add to a specific volume of water, or how much water you need to fill a pool or fish tank. To make this calculation, you first need to figure out the square footage of the tank, and then multiply that by the constant number of gallons per square feet.

Measure the width of the tank with a measuring tape. If your tank is a rectangle, also measure the height and depth.

Multiply its width by its height by its depth. The result is the cubic feet of space the tank holds.

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Multiply the result by 7.48. The answer is the number of gallons of water the tank holds. 7.48 is a the number of gallons in a square foot of space.