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Typical Building Footprint example:

Tank Sizing Calculator

The tank sizing calculator is for estimating purposes only and uses the potential catchment footprint being utilized for Rainwater Harvesting, Total Annual Rainfall (inches) based on the National Climate Data Center Normal Monthly Precipitation Chart supplied here: http://average-rainfall., the Runoff Coefficient of the roof or catchment area and finally a safety factor to account for first flush diversion, evaporation and/or freezing in cold climates.


Enter your roof or catchment area dimensions starting first with length in feet.



Enter your roof or catchment area dimensions width in feet. (Note: If you have a complicated non-linear area, take the square root of the total area and enter that number for both Step-1 (length in feet) and Step-2 (width in feet). Example total roof area equals 3,458 square feet, you would take the square root of 3,458 and come up with 58.80 feet for both length & width of your catchment area.



Enter a Runoff Coefficient using the table referenced below:
(Typical range: 0.70-0.95) See table below for the range of values



Enter your cities total average annual rainfall amount in inches Please review note below regarding finding your cities average annual rainfall amount.



Enter a safety factor (Typical range: 0.65-0.95)
See info below regarding safety factor



Disclaimer, this Rainwater Harvest Calculator is for general estimation purposes only. It does not incorporate water loss due to evaportranspiration or consider individual water requirements. Please contact CalWater Solutions for an on-site assessment to ensure you fully understand the results provided by this tool.

Runoff coefficient ranges here:

Runoff Coefficient
Surface Character of surface High Low
Roof Metal 0.98 0.95
Roof Asphalt Shingle 0.95 0.70
Roof Gravel 0.75 0.70
Roof Tile 0.85 0.70
Roof Green < 4″ depth 0.55 0.50
Roof Green > 4″ depth 0.50 0.30

Average Annual Rainfall for your city:

Find your cities average annual rainfall amount in inches by clicking on the following link and typing your city name in the magnifying glass under “City” not “All Topics”: Scroll down to where you can see a graphical representation of your cities Monthly Rainfall. Above the chart you your cities total average annual rainfall (i.e. Berkeley has a total average annual rainfall of 23.96), which can be rounded up to the nearest inch.

Data provided by National Oceanic and Atmospheric Administration:

Visit for more information.

Safety Factor Notes:

In the planning process for Rainwater Harvesting a Safety Factor is used to underestimate the potential for rainfall catchment. By underestimating the potential, the planner ensures that enough water will be available. This is critical for a family or business that relies solely on rainwater for drinking or irrigating food crops. When determining a safety factor, examine the potential for leakage, pluggage, spillage and other factors that reduce the efficiency of a collection system. Also take into consideration variations in rainfall. For example, an area that is prone to intense rainstorms may have a lower design safety factor than in other areas because less wind-blown rain may land on the roof. Consider a design safety factor that ranges between 0.95 and 0.65.