Quick answer: Harvestable rainwater V = A × R × C, where A is roof catchment area (m²), R is annual/peak rainfall (m) and C is the runoff coefficient (0.8–0.9 for RCC roofs). Tank and recharge pit are then sized from the peak-event volume and desired storage days.
How to calculate harvesting potential
Volume (litres) = Area (m²) × Rainfall (mm) × Runoff coeff
Runoff coefficient: RCC roof 0.85, tiled 0.75, unpaved 0.2
Recharge pit sized from peak hourly intensityWorked example
Roof 300 m², annual rainfall 1000 mm, C = 0.85: V = 300 × 1000 × 0.85 = 2,55,000 litres/year harvestable. A storage tank is sized from the design storage period; the recharge pit from peak rainfall intensity.
Roof catchment potential, tank and recharge pit sizing to NBC Part 9. Free tool.
Open the Rainwater Harvesting Calculator →Frequently Asked Questions
How do you calculate rainwater harvesting potential?
Multiply the roof catchment area by the rainfall depth and the runoff coefficient. For an RCC roof the runoff coefficient is about 0.85. The result is the harvestable volume for that rainfall.
Is rainwater harvesting mandatory in India?
Many Indian states and municipal bye-laws mandate rainwater harvesting for plots above a certain size, and NBC Part 9 provides the design framework. Check local rules for the exact threshold.