Rainwater Collection Calculator

Estimate roof runoff collection

Enter horizontal roof catchment area, rainfall depth, and an efficiency allowance for first-flush diversion, splash, leaks, and incomplete capture. One inch of rain on one square foot yields approximately 0.623 US gallons before losses.

A 500-square-foot roof receiving one inch of rain has a theoretical yield of about 311.5 gallons. At 85% efficiency, approximately 264.8 gallons may be collected.

Use the correct roof area

Use the roof’s horizontal footprint rather than sloped surface area. If only part drains to the collection system, include only that section. Local rainfall reports can help estimate seasonal potential.

Water quality matters

Roof materials, wildlife, dust, smoke, gutters, and storage conditions affect water quality. Use screens and first-flush management, keep tanks covered, and follow local requirements. Untreated rainwater should not be assumed safe for drinking or sensitive uses.

System planning

• Provide safe overflow routing.
• Protect tanks from light, pests, and contamination.
• Account for dry periods and irrigation demand.
• Check local collection regulations.

This is a volume estimate. Actual yield and suitability depend on the catchment, weather, maintenance, treatment, and storage system.