The cistern size depends on intended use of the water, and the distribution of rainfall throughout the year. A margin of 1/3 is allowed for water lost from leaking pipes, evaporation, wind, etc. For example, to provide 100 gallons of water per day, in an area where rainfall is limited to only one season, a very large cistern is needed with a storage capacity of 30,000 gallons. In a region with rainfall distributed throughout the year, a 3,000-gallon cistern may be needed.

The catchment area and cistern should be sized simultaneously. The catchment area needs to be large enough to collect sufficient amounts of rain.

The formula for calculating the amount of water collected is:
Gallons = 0.416 (Rainfall in inches) (Area in square feet)

To calculate the catchment area necessary for collecting a specific amount of water:
Area = 2.41 (Gallons)(Rainfall in inches)
(Calculations can be found at http://edis.ifas.ufl.edu)

In a typical rainwater collection installation, a building roof is needed to provide the catchment area. The cistern is then located near this catchment. Distance from trees will decrease the possibility of roots causing problems of cracking, and a pretreatment system will help keep out debris, leaves, insects, and bird droppings. A chlorination system is necessary if the water is to be potable. The quality of rainwater varies depending on pollution in the area.

Cisterns should not be located near sewage lines or any other possible sources of contamination. In order to drain surface water away from the cistern, it should be located on a sloped area. A firm ground surface helps avoid the potential for settling which can lead to cracking in the cistern.

Cistern components should be maintained. Areas of initial water reception should be kept free of leaves and debris. Manhole covers should be kept tight and leaks repaired quickly.