If your tank has to be filled regularly with some substance, it is important to know the volume of that tank, as well as the gallons of liquid that it hold. Understanding these calculations helps to better estimate how much water or fuel needed, as well as when to fill the tank again. Determining the volume of a tank and the amount of gallons that it contains is as simple as performing a few measurements and calculations.

### Rectangular Tanks

Use a tape measure to measure the length of the tank. Measure the width and depth of the tank as well.

Multiply the length, width and depth measurements in order to find the cubic volume.

Divide the cubic volume in inches by 231, as a gallon is equal to 231 cubic inches.

### Cylindrical Tanks

Measure the 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 (width) by two.

Measure the height of the tank.

Square the radius by multiplying the radius times itself.

Multiply the answer from step three by 3.14 (pi), and then multiply that product by the height in order to find the cubic volume. Divide the cubic volume in inches by 231 in order to find the number of gallons.