Instructions
Measure the length, width, and height of your tank.
Multiply the length by the width by the height (capacity equals length times width times height), using a calculator. This is the total capacity of your tank in cubic units. For example, if your tank is 24 inches long, 12 inches wide, and 18 inches high, you will multiply 24 times 12 times 18 to get 5,184 cubic inches.
Convert cubic inches to gallons. It's helpful to know your tank's capacity in gallons. To do this, divide the cubic inches by 231. For the above example, use your calculator to find 5,184 divided by 231. You get 22.4 gallons. This is the total capacity of your tank in gallons.
Calculate your tank's capacity for the number of fish you can keep. Take your height measurement and subtract the amount of gravel at the bottom of your tank and the amount of airspace at the top. For example, if you have 2 inches of gravel at the bottom of your tank and 3 inches of airspace at the top of your tank, subtract 4 inches from the height of your tank. Therefore, if your tank is 18 inches tall and you subtract 4 inches for gravel and airspace, use 14 as your height. Multiply this adjusted height figure to the width and depth of your tank to find out how much water capacity you have for fish. Convert this number to gallons.
Figure out how many fish you can safely keep in your tank. A good rule of thumb is that you can keep 1 inch of fish per gallon of water. Therefore, in a tank that holds 20 gallons, keep 20 inches of fish. You could keep 10 2-inch fish, four 5-inch fish, or any other combination of fish that add up to 20 linear inches. When figuring out how many fish you can keep, consider their full-grown length, even if they are babies now. You don't want to have to get rid of fish later because they have outgrown their tank.
How to Calculate Tank Capacity
It's important to know your tank capacity for a variety of reasons. When purchasing aquarium gravel, tank filters, or heaters, knowing the capacity is key. It's also important to know your tank's capacity so you don't buy too many fish for the available space. All you need is a tape measure and a calculator to figure out the capacity.