Rainwater Harvesting Calculator
How roof runoff becomes a water supply
Turning storms into a dependable water source starts with understanding how much precipitation lands on your roof each year. This calculator multiplies roof area, annual rainfall, a conversion factor, and your system’s collection efficiency to compute the volume you can realistically capture. The simple arithmetic hides practical considerations like debris filters, first-flush diverters, and seasonal rainfall patterns, so the explanatory notes help you interpret the output responsibly.
The rain-to-gallons conversion is rooted in geometry: one inch of rain falling on a square foot of roof equals roughly gallons. The harvested volume follows the relationship
where is roof area in square feet, is annual rainfall in inches, and is capture efficiency expressed as a decimal. Dividing by twelve reveals average monthly availability, while multiplying by 3.785 converts gallons to liters for metric comparisons. Use the copy button to record results in design notes or share them with contractors.
Illustrative harvest scenarios
Annual rainfall varies dramatically by location. The comparison table shows how three climates affect the same 1,200 ft² roof at 80% efficiency. Adjust the calculator inputs to match your weather records and see how the numbers shift.
| Climate | Rainfall (in/year) | Harvest (gal/year) | Average (gal/month) |
|---|---|---|---|
| Arid | 12 | 7,177 | 598 |
| Temperate | 30 | 17,943 | 1,495 |
| Tropical | 60 | 35,886 | 2,990 |
These values assume consistent rainfall, yet real weather arrives in bursts. When storms cluster in a rainy season, storage sizing becomes critical. Combine this annual view with the Rain Barrel Sizing Calculator or Rainwater Cistern Sizing Calculator to ensure you can bridge dry spells.
Planning, budgeting, and next steps
Once you know your annual harvest potential, compare it with irrigation demand, greywater substitution goals, or livestock needs. If you are evaluating system payback, plug the annual gallons into the Rainwater Harvesting Payback Calculator to contrast equipment costs with utility savings. Maintenance matters too—clean gutters, inspect screens, and monitor first-flush diverters to keep efficiency close to your assumption.
Documenting these calculations supports permit applications and community presentations alike. Rainwater harvesting reduces stormwater runoff, protects watersheds from erosion, and provides resilience during drought restrictions. With quantified expectations, you can advocate for rebates or grants that offset installation costs while demonstrating environmental benefits.