Rainwater Harvesting Calculator
Find how much rainwater a roof can collect in a year.
About the Rainwater Harvesting Calculator
Enter the roof area, yearly rainfall and the roof's runoff coefficient. The calculator gives the rainwater you could collect each year in litres and gallons.
Every millimetre of rain on a square metre is one litre. The runoff coefficient accounts for water lost to evaporation, splash and first-flush diversion: about 0.8–0.9 for metal and tile roofs. Size storage on the driest months' needs, not the yearly total.
The formula
Litres = area (m²) × rainfall (mm) × runoff coefficient
Worked example
A 1,500 ft² metal roof (coefficient 0.8) with 38 inches of rain a year
- 1,500 × 38 ÷ 12 = 4,750 ft³ of rain
- × 0.8 = 3,800 ft³ collected
- = 107,604 L (28,426 US gallons)
Questions people ask
Is rainwater drinkable?
Not without proper filtration and treatment; use it for gardens, toilets and laundry.
Is harvesting legal?
In most places yes; a few US states regulate it, and some Indian cities require it for new buildings.