How it is calculated
Each inch of rain on 1 sq ft of roof gives about 0.62 gallons. Gallons per year = roof area × rainfall × 0.623 × runoff coefficient (about 0.8 to 0.9 for most roofs).
Example
A 1,500 sq ft roof with 38 in of rain a year collects about 28,400 gallons a year.
These are estimates to plan your purchase. Check measurements on site and follow the manufacturer’s instructions and local building codes.
Frequently asked questions
Is collecting rainwater legal?
In most US states yes, but a few have limits or rules. Check your state before installing a large system.
See also
Results are estimates for information only and are not financial, legal or medical advice. Rates, taxes and fees vary — confirm with a professional or your lender.