Rainwater Harvesting Calculator
Estimate collection potential and a dry-period storage scenario. Annual totals cannot establish how much rainwater will be available when you need it.
Uses 144/231 US gallons per square foot per inch of rain. Collection efficiency is an illustrative assumption. The annual demand and bill figures are upper bounds before rainfall timing, tank overflow and pumping costs. Storage assumes a full tank at the start of the entered dry period.
How the math works
gallons/year = roof sq ft × rainfall inches × 0.623 × efficiency
Frequently asked questions
How much rain reaches a roof?
One inch over one square foot equals 144 cubic inches. Dividing by 231 cubic inches per US gallon gives about 0.623 gallons. The efficiency input represents your assumed collection losses.
Does annual rainfall determine the tank size?
No. Storage design needs the timing of rain, demand, overflow and drought. This tool shows a storage scenario for the dry period you enter, assuming the tank begins full.
Which roof area should I enter?
Use the horizontal footprint of the roof section draining to the collection system, rather than the sloped surface area.
Is collected rainwater safe to use?
Roof runoff can contain germs and chemicals. CDC recommends using safe tap or bottled water for drinking, preparing food and watering edible plants when possible. Confirm appropriate uses, treatment and local requirements with your health department.
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