Rainwater Harvesting Volume Calculator

How to calculate Rainwater Harvesting?

To determine how much water you can collect, you need to look at the total footprint of your collection area, the amount of rainfall your region receives, and how efficiently your roof surface moves that water into your gutters.

Volume (Gallons) = Area (sq ft) × Rainfall (in) × 0.623 × Runoff Coefficient

Using the Harvest Calculator: an example

Consider a home with a 2,000 sq ft metal roof located in an area that receives 40 inches of rain annually.

Step-by-step calculation:

1. Calculate raw potential: 2,000 sq ft × 40 in × 0.623 = 49,840 gallons.
2. Apply runoff coefficient: 49,840 × 0.95 (metal roof) = 47,348 gallons.
3. Factor in system efficiency: 47,348 × 0.80 = 37,878 usable gallons.

Frequently Asked Questions

Is rainwater safe to drink? While naturally pure, rainwater can pick up contaminants from the roof; it requires filtration and UV treatment to be safely potable.

What size tank do I need? Your tank size should be based on your monthly water demand versus your collection potential, typically sized to last through dry spells.

What is a runoff coefficient? This is a decimal representing the efficiency of your roof material; for example, metal (0.95) sheds more water than asphalt shingles (0.85).