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About Rainwater Harvesting Calculator
Estimate annual and monthly rainwater collection potential from roof catchment areas, applying material-specific runoff efficiency and first-flush diversion to size storage cisterns.
Inputs and calculations stay in your browser and require no signup.
How the calculation works
Planning calculation
Annual yield (gallons) = catchment area (ft²) × annual rainfall (in) × 0.6233 gal/ft²-in × runoff coefficient × (1 − filter loss / 100); Recommended tank capacity = (annual yield / 365) × dry-period storage days.
How to use this calculator
- 1Enter measured project dimensions and quantities.
- 2Adjust material, spacing, waste or density assumptions to match the project.
- 3Review the live planning quantities.
- 4Round purchasing quantities appropriately and confirm field conditions.
Understanding the result
1 inch of rainfall over 1,000 square feet of roof yields approximately 623 gallons of water before efficiency deductions. Metal roofs provide the highest catchment efficiency (0.95), followed by glazed tile (0.85), flat membrane (0.85), and asphalt shingles (0.80).
Results use the units shown beside each field and output.
Important limitations
- Rainfall distribution is seasonal; monthly rainfall totals vary significantly from uniform annual averages.
- Potable drinking use requires certified sediment filtration, sub-micron carbon block filters, and UV disinfection.
- Verify local municipal codes and water rights regarding residential rainwater harvesting cisterns.
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Frequently asked questions
How much rainwater can I collect from a 2,000 sq ft roof?
In an area with 35 inches of annual rainfall, a 2,000 sq ft metal roof yields approximately 39,000 to 41,000 gallons (approx 150,000 liters) of clean harvestable water per year.
Why does roof material affect rainwater catchment yield?
Different materials have different absorption and friction factors. Smooth metal roofs lose only ~5% of water to evaporation/texture, while porous asphalt shingles absorb water and release loose granules, losing ~20%.
How large should my rainwater collection tank be?
For landscape irrigation, size your cistern to store 2 to 4 weeks of dry-spell demand. For whole-home non-potable toilet and laundry use, standard cisterns typically range between 1,500 and 5,000 gallons.
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