When it comes time to fill a rectangular tank, whether it’s an aquarium or a swimming pool, the number of gallons the task will take is usually a mystery. That’s probably because most of us don’t have a very good sense of how much space a gallon takes up. The simple way to calculate how much water (or any other liquid) a tank can hold is to measure the tank's volume and convert that figure to gallons.

Measure the length, width, and depth of the interior of the tank in feet and inches. Convert the dimensions to decimal feet by dividing the number of inches by twelve and adding to the number of feet. For example, 7 feet 9 inches is 7.75 feet.

Calculate the tank’s volume in cubic feet. This is the product of the length, width and depth. For example, 7.75 feet long by 4.5 feet wide by 3 feet deep would be 7.75 * 4.5 * 3 = 104.625 cubic feet.

Convert the volume in cubic feet to the capacity in gallons. Multiply the tank’s volume by 7.48, the number of gallons in a cubic foot. For example, 104.625 * 7.48 = 782.6 gallons.