Your fish tank's capacity -- how much water it can hold -- can greatly affect the growth and health of your fish. You need to keep tank capacity in mind when buying aquarium-care supplies and when determining how many fish, and what size fish, your tank can safely accommodate. Capacity, or volume, generally is expressed in gallons or litres. If you do not happen to know your tank's capacity, you can easily calculate it.

### Rectangular tank

Measure the length, width and height of the inside of your tank with a tape measure in cm. Multiply those three measurements to determine the capacity, or volume, of the tank in cubic centimetres.

Determine the volume of the tank in litres using the fact that 1 cubic cm = 1 ml. This means 1,500 cubic cm = 1,500 ml or 1.5 litres. There are 1,000 ml in one litre.

Multiply the volume figure (in cubic cm) by 0.000219969 if you want to determine the number of gallons.

### Cylindrical tank

Measure across the top (the diameter) of the tank and also measure the height, from the bottom to the top edge in cm, using a tape measure. Divide the diameter in half to determine the radius. Calculate the capacity, or volume, using the following formula: pi (3.1416) times the radius squared times the height.

Use the fact that 1 cubic cm = 1 ml to determine the capacity of the tank in litres, as in the previous section.

Multiply the volume figure (in cubic cm) by 0.000219969 to determine the number of gallons.

### Bowl

Measure inside your fish bowl, across its widest point, in cm with a tape measure; this measurement is the diameter. Divide the diameter in half to determine the radius. Calculate the capacity, or volume, using the following formula: 4/3 times pi (3.14) times the radius cubed.

Determine the volume of the tank in litres using the facts that 1 cubic cm = 1 ml and 1,000 ml = 1 litre.

Multiply the volume measurement (in cubic cm) by 0.000219969 to determine the number of gallons.