Volume (V) of a rectangular tank is calculated by multiplying measurements of the length (l), width (w), and depth (h):

V = l × w × h

Example: A rectangular tank is 30 cm long, 12 cm wide, and the water is 20 cm deep.

V = l × w × h

= 30 cm × 12 cm × 20 cm

= 7,200 mL

= 7.2 L

Volume (V) of a cylindrical tank is calculated by multiplying the area of the base (π times the square of the radius [r]) by the depth (h):

V = (π × r²) × h

Example: A cylindrical tank has a radius of 10 cm, and the water is 20 cm deep.

V = (π × r²) × h

= (3.14159 × [10 cm]²) × 20 cm

= 6283.18 mL

= 6.28 L

Conversion factors:

1 gal. = 231 cubic inches

1 gal. = 3.785 L

1 L = 0.264 gal.

1 cubic foot = 7.481 gal.

If the container is oddly shaped, volume may be calculated mathematically, but it may be easier to purchase a flow meter to measure the volume required to fill the tank. Alternatively, the volume of inflowing water per minute can be measured by determining how long it takes to fill a 1-L graduated cylinder (or 5-gal. bucket). Using that information, determining how long it takes to fill a tank or ornamental pond can provide a sufficiently accurate assessment of volume.