Rainwater Harvesting Calculator

Enter your property address or select your parcel on the map to get personalized rainwater collection estimates

Enter Property Address

How Does This Calculator Work?
  • Site-specific rainfall: Uses PRISM precipitation daily data from 1983 to 2024 to estimate average monthly rainfall at your coordinates. You can also switch to historical simulations for drought realism.
  • Catchment yield: For each month, harvestable volume is computed as
    Yield = Rain (in) × Roof Area (ft²) × 0.623 × E
    where 0.623 is the constant conversion factor to turn inches of rainfall per square foot into gallons and E is capture/collection efficiency (default 0.85).
  • Demand model: Indoor demand is applied year-round; outdoor demand is applied for 7 months (Mar–Sep by default) to reflect seasonal irrigation.
  • Storage simulation: The model steps through time, adds collected volume, subtracts demand, and bounds storage between 0 and the tank’s capacity. Starting fill defaults to 25% but can be set by the user after the initial analysis run.
Typical Roof Runoff Coefficients

Use these as inputs for E; they represent the fraction of rainfall that actually leaves the roof and reaches the conveyance system.

  • Metal: 0.92
  • Polycarbonate plastic: 0.91
  • Asphalt shingles / concrete: 0.90
  • Tar & gravel: 0.80–0.85
  • Flat gravel: 0.62
  • Clay tile: 0.50
  • Green roof: 0.23

(Values summarized from published studies and guidance; actual performance varies with slope, condition, debris, and maintenance. If you have a rain gauge and meter, you can estimate your site’s E directly.)

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