How it works
One inch of rain over one square foot is about 0.623 gallons. So multiply roof area in square feet by rainfall in inches and by 0.623, then multiply by collection efficiency as a decimal. In plain words: gallons = roof area × rainfall × 0.623 × efficiency. For example, 1,000 sq ft × 1 inch × 0.623 × 0.85 = 529.55 gallons. Efficiency accounts for water lost to splashing, evaporation, gutter overflow, and first-flush diverters.
Frequently asked questions
Should I use the flat roof area or the footprint?
Use the horizontal footprint area that catches rain, not the sloped surface area. If you only know the sloped area, your result will be a little high.
What efficiency should I enter?
Enter your own estimate for your roof and gutters. This calculator does not claim a standard rate — losses depend on the system, so a conservative figure gives a safer estimate.
Can I drink this water?
Not on the basis of this calculator. Treatment, filtration, storage, and local rules all matter. Get local water-safety guidance before using collected water for drinking.