Rainwater Harvesting Calculator
A 200 m² concrete roof under 800 mm of annual rain yields 122400 L a year after 10 % first-flush losses.
RCC or GI roof 0.8–0.9, paved 0.6–0.8, lawn 0.1–0.3.
122400 L
- In cubic metres
- 122.4 m³
- Days of demand met
- 244.8 days
- Storage tank for the dry spell
- 30000 L
Step-by-step solution
- V = A × R × C × (1 − losses) = 200 m² × 0.8 m × 0.85 × 0.9 = 122.4 m³
Rainwater Harvesting Formula
V = A × R × C
A roof area (m²), R rainfall (m), C runoff coefficient
Worked Example
A 200 m² roof with 800 mm of rain a year:
- V = A × R × C × (1 − losses) = 200 m² × 0.8 m × 0.85 × 0.9 = 122.4 m³
Days of demand met: 244.8 days · Storage tank for the dry spell: 30000 L
GATE tip
1 mm of rain on 1 m² is exactly 1 litre: the quickest way to sanity-check the answer.
More Services and Infrastructure Calculators
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Water Demand Calculator
Q = P × q / 10⁶ MLD
peak factor 3.0 (< 50,000), 2.5 (to 2 lakh), 2.0 (to 20 lakh), 1.8 (larger)
Example: A town of 1,00,000 at 135 lpcd: Average daily demand = 13.5 MLD.
Frequently asked questions
How big should the tank be?
Enough for the longest dry spell’s demand, but no larger than the yearly yield.
What is the first flush?
The first few millimetres of each storm, which wash dust and droppings off the roof and are diverted to waste.
Sources
- The International System of Units (SI), 9th edition (SI Brochure), Bureau International des Poids et Mesures .
Last reviewed: . Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.
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APA
Scientific Converters. (2026, October 15). Rainwater Harvesting Calculator. Retrieved October 29, 2026, from https://scientificconverters.com/calculators/rainwater-harvesting-calculator/
IEEE
Scientific Converters, "Rainwater Harvesting Calculator," scientificconverters.com. Accessed: Oct. 29, 2026. [Online]. Available: https://scientificconverters.com/calculators/rainwater-harvesting-calculator/