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Event Rainwater Harvesting Calculator — How It Works

Event Rainwater Harvesting Calculator

Event Rainwater Harvesting Calculator — How It Works

Estimates how much rainwater can be collected from a roof during a rainfall event.

How it works

  1. Enter your roof’s catchment area in square feet.
  2. Enter the rainfall amount in inches.
  3. Enter your collection system’s efficiency — typically 75-90%, accounting for losses from evaporation, overflow, and first-flush diversion.
  4. The tool calculates how many gallons (and liters) of water you could collect, and how many 55-gallon barrels that would fill.

Formula

Water (gallons) = Roof area (sq ft) × Rainfall (in) × 0.623 × Efficiency

Example

A 1,000 sq ft roof during 1 inch of rainfall, at 85% efficiency, collects about 530 gallons (2,005 liters) — enough to fill roughly 10 standard 55-gallon barrels.

Notes

The constant 0.623 converts square-foot-inches of rainfall to gallons. Actual collected volume depends on your gutter and downspout setup, roof material, and how much “first flush” (the initial, dirtiest runoff) you divert before storage.

Try the Event Rainwater Harvesting Calculator — instant results in your browser, no sign-up.

FAQ

Is this free?

Yes — the Event Rainwater Harvesting Calculator runs entirely in your browser, no sign-up required.

Are these figures exact?

The calculator performs the math precisely from the numbers you enter, but real-world rates, emissions factors, and efficiency figures vary by location and equipment — treat results as planning estimates.