Rainwater Harvesting Calculator

Example

Input: 100m² Roof, 50mm Rain

Result: 4,500 Liters

Step-by-Step Guide

1. Measure Roof

Calculate the footprint area of the roof (Length x Width). Slope doesn't increase catchment area significantly for rain calcs.

2. Input Rainfall

Enter rainfall amount in mm (e.g., average monthly rainfall).

3. Calculate

Multiply Area * Rain * 0.9.

4. Convert

Result is in Liters (1000L = 1m³).

What is Rainwater Harvesting Calculator?

Estimates the volume of water that can be collected from a catchment area (roof) during a rainfall event. Essential for sustainable gardening and off-grid water systems.

How it Works

Formula: Roof Area (m²) × Rainfall (mm) × Efficiency Coefficient = Liters Collected. Typically, a coefficient of 0.85 to 0.90 is used to account for evaporation, splash-out, and first-flush diversion.

FAQ

Does roof pitch matter?

No. The catchment area is the building's footprint (horizontal projection), regardless of slope.

Is the water drinkable?

Not without proper filtration and treatment (potable). It is great for irrigation.

What is a first-flush diverter?

A device that discards the first few liters of dirty roof wash before filling the tank.

Gallons conversion?

1mm rain on 1 sq ft = ~0.623 gallons.

Metal vs Tile roof?

Metal has a higher efficiency coefficient (>0.9) than tile or concrete (<0.8).

Conclusion

Even a small roof can capture massive amounts of water. A 100m² roof in a 50mm rain event yields enough water to fill twenty 225-liter barrels. Size your tank based on your region's longest dry spell, not just a single storm.

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References & Standards

This calculator uses formulas and data standards from Standard References to ensure accuracy.

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