How to use the rainwater harvesting calculator
- Enter your roof area as seen from above (length × width).
- Enter your annual rainfall — or one month’s rainfall to see a monthly figure.
- Choose your roof surface (or enter your own runoff coefficient).
- Optional: enter the number of people and how much rainwater each uses per day to see how many days of supply you collect.
Rainwater harvesting formula
Water harvested = Catchment area × Rainfall × Runoff coefficient Metric: Litres = Area (m²) × Rainfall (mm) × Coefficient Imperial: Gallons = Area (ft²) × Rainfall (in) × 0.623 × Coefficient
The handy rule: 1 mm of rain on 1 square metre = 1 litre, and 1 inch of rain on 1,000 square feet ≈ 623 US gallons.
Worked example
A house in a city with 800 mm of rain a year has a 100 m² concrete terrace (coefficient 0.8):
- 100 × 800 × 0.8 = 64,000 litres a year (about 5,300 litres a month on average)
- A family of four using 50 litres each a day for flushing and garden: 64,000 ÷ 200 = 320 days of supply
Annual rainwater from common roof sizes
| Roof area | 600 mm rain | 800 mm | 1,200 mm | 2,000 mm |
|---|---|---|---|---|
| 50 m² (538 ft²) | 24,000 L | 32,000 L | 48,000 L | 80,000 L |
| 100 m² (1,076 ft²) | 48,000 L | 64,000 L | 96,000 L | 160,000 L |
| 200 m² (2,153 ft²) | 96,000 L | 128,000 L | 192,000 L | 320,000 L |
Values use a runoff coefficient of 0.8.
Where to find your rainfall
In India, the India Meteorological Department (IMD) publishes normal annual and monthly rainfall for every district. In the US, NOAA climate normals list average monthly precipitation for thousands of weather stations, and most countries’ weather services publish similar averages. For planning a tank, look at monthly figures: most of India’s rain falls in the four monsoon months, so a large share of the year’s water arrives in a short time, while the dry months need stored water.
Storage or recharge?
There are two ways to use harvested rain. Storage sends roof water through a filter to a tank or sump for later use. Groundwater recharge sends it into a recharge pit, trench or old borewell to refill the aquifer. Many cities in India now require rooftop rainwater harvesting for new buildings, often with a recharge pit. A good system usually does both: store what you can use and recharge the overflow.
Tips for a good system
- Fit a first-flush diverter to discard the dirty first few millimetres of each rain.
- Use mesh and a filter to keep out leaves, insects and debris.
- Keep tanks covered and dark to stop algae and mosquitoes.
- Clean the roof and gutters before the monsoon or rainy season.
- Check local rules and subsidies — many municipalities offer rebates or mandate harvesting.