Rainwater Harvesting Calculator
See how much rainwater your roof can collect in a year and what size storage tank or recharge pit you need.
Inputs
Formula
- Harvest = roof area × rainfall × runoff coefficient
- 1 m² × 1 mm of rain = 1 litre
- Storage ≈ harvest from the largest rain spell (about 20 days of rainfall)
Every square metre of roof collects one litre for each millimetre of rain, minus losses from splash, evaporation and the first flush. A concrete terrace keeps about 85% of it. Compare the annual harvest with your family's demand to see how many months of water the roof can supply.
Step-by-step calculation
- 1
Roof area
sq ft × 0.0929
111.48 m²
- 2
Harvest
area × rainfall × 0.85
85,285 litres
- 3
Family demand
people × 135 × 365
197,100 litres
Worked example
- 11,200 sq ft = 111.5 m² of roof, 900 mm annual rainfall.
- 2Harvest = 111.5 × 900 × 0.85 = 85,300 litres a year.
- 3A family of 4 uses about 197,000 litres a year, so the roof covers 43% of the demand.
Tips
- Fit a first-flush diverter — the first 2 mm of rain carries all the roof dust.
- If you cannot store it, recharge it: a 1 m × 1 m × 2 m pit filled with gravel recharges the borewell.
Frequently asked questions
How much rainwater can a 1000 sq ft roof collect?
About 71,000 litres a year in a place with 900 mm rainfall — enough for 130 days of a family of four.
Is rainwater safe to drink?
Only after filtration and disinfection. Most homes use it for washing, flushing and gardening, or recharge it into the ground.
What tank size do I need?
Storing the harvest from one heavy spell is enough — typically 5,000–10,000 litres for a house.
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