Determining the volume of water in litres starts with measuring the tank and producing a cubic inch measurement. From there, a simply conversion allows the resulting cubic inch volume to be presented in litres. Most fish tanks are rectangular, although some other tanks, such as water towers, might be cylindrical. The calculation required is dependant on the type of tank in question.
Measure the length, width and height of the full tank in inches. If the tank is not full, then the height will be the height of the water level.
Multiply the values together to calculate cubic inches. As an example, if you measured 12 inches by 36 inches by 20 inches, the total volume would calculated as 12 times 36 times 30, or 8,640 cubic inches.
Multiply the cubic inches by 0.01639 to calculate litres. In the example, 8,640 times 0.01639 equals 141.6 litres.
Measure the diameter and height of the full tank. The diameter is the distance across the circular top or base measured in a line across the centre. If a vertical tank is not full, you can use the height of the water level. For horizontal tanks, you can turn the tank vertically and measure the water height, if feasible.
Divide the diameter by two to find the radius. As an example, a tank with a diameter of 36 inches would have a radius of 18 inches.
Square the radius and multiply by the height and then 3.14. In the example, if the height was 48 inches, then the total volume would be calculated as 18 times 18 times 48 times 3.14, or 48,833 cubic inches.
Multiply the cubic inches by 0.01639 to calculate litres. In the example, 48,833 times 0.01639 equals 800.4 litres.
If you already have the volume in gallons, you can simply multiply the number of gallons by 3.79 to convert the volume into litres.