Guidance on use of rainwater tanks

A. Determining the size of installed tanks

Page last updated: March 2011

Tank sizes range from small modular tanks of 750 L (~165 gallons) to over 50 000 L (~11 000 gallons). To convert a volume in gallons to a volume in litres, multiply the number of gallons by 4.5.

Methods to calculate the volume of tanks are given below.


To calculate the volume of a rectangular tank, use the formula:
Volume (in litres) = depth (cm) x width (cm) x length (cm) 1000
For example, the volume of a 1.6m by 0.5m by 2.4m tank in litres would be:
Volume = 160 x 50 x 240 1000 = 1920 L


To calculate the volume of a cylindrical tank either use the formula:
Volume (in litres) = p x diameter (cm2) x depth (cm) 4000
(p = 3.142)

For example, the volume in a tank with a diameter of 2m and a depth of 2.6m would be:
Volume = 3.142 x 200 x 200 x 260 4000 = 8169 L