If you don't know how many gallons a tank can hold, you can calculate it yourself using a simple formula. The measurements you need to calculate the volume are different, depending on the tank you have. Rectangular tanks, like aquariums require one formula, while cylindrical tanks, like drinking water tanks, require another.
Measure the length, width and depth of the tank using a tape measure. You can measure from the inside of the tank or, if preferable, measure the outside and then subtract the width of the tank walls to determine the inside measurement.
Next, multiply the length, width and depth measurements in order to find the cubic volume in cubic inches. For example, if the measurements are 20 inches by 12 inches by 12 inches, the volume is 2,880 cubic inches.
Finally, divide the cubic volume in inches by 231, as a gallon is equal to 231 cubic inches. A 2,880 cubic-inch tank is 12.47 gallons
Measure the height and radius of the tank. The radius is the distance from the center of the tank to its outer edge. Another way to find the radius is to divide the diameter, or width, by two.
Square the radius by multiplying the radius times itself and then multiply it by 3.1416, which is the constant pi. Multiply this number by the height to determine the cubic volume in inches. For example, if you have a tank that is 12 inches in diameter and 20 inches high, the calculation looks like this:
Divide the cubic volume in inches by 231 in order to find the number of gallons.
TL;DR (Too Long; Didn't Read)
If you want to measure larger tanks in feet, multiply the cubic feet by 7.48 to convert cubic feet to gallons. The precise conversion factor is 7.48052.