Roof Water Collection Calculator – Calculate Rainwater Harvesting Potential

What is a Roof Water Collection Calculator?

A roof water collection calculator helps estimate how much rainwater you can collect from your roof based on rainfall, roof area, and material efficiency. This is especially useful for planning rainwater harvesting systems for homes, offices, or farms.

Main Formula

The basic formula for calculating total annual rainwater collected is:

$\text{Collected Water Volume}=\text{Roof Area}\times \text{Rainfall}\times \text{Runoff Coefficient}\times \text{Collection Efficiency}$

1. Roof Area

The total horizontal area of the roof (in square meters).
If your roof has multiple sections, sum all their areas.

$A=L\times W$

2. Rainfall

Average annual rainfall in millimeters (mm).

To convert rainfall (mm) into meters for volume calculation, divide by 1000.

$R=\frac{\text{Rainfall (mm)}}{1000}$

3. Runoff Coefficient (C)

This depends on the roof material:

4. Collection Efficiency (E)

Account for system losses — gutters, filters, evaporation, etc.
Typical range: 80–95% (0.8–0.95)

Full Formula with Units

$V=A\times R\times C\times E$

Where:

• V = Volume of water collected (m³)
• A = Roof area (m²)
• R = Rainfall (m)
• C = Runoff coefficient
• E = Efficiency (decimal)

Convert Volume to Liters

Since 1 cubic meter = 1000 liters:

$V_{\text{liters}}=V\times 1000$

Example Calculation

Example:
Suppose you have:

• Roof area = 100 m²
• Annual rainfall = 800 mm
• Roof material = Concrete (C = 0.85)
• Efficiency = 90% (0.9)

Step 1: Convert rainfall to meters

$R=\frac{800}{1000}=0.8\text{ m}$

Step 2: Use the main formula

$V=100\times 0.8\times 0.85\times 0.9$

Step 3: Convert to liters

$V_{\text{liters}}=61.2\times 1000=61,200\text{ liters}$

Result:
You can collect approximately 61,200 liters of rainwater per year from a 100 m² roof in a region with 800 mm annual rainfall.

Optional Calculation (Monthly Average)

$V_{\text{monthly}}=\frac{V_{\text{liters}}}{12}$

$V_{\text{monthly}}=\frac{61,200}{12}=5,100\text{ liters/month}$

Conclusion

Using this simple formula, you can estimate how much rainwater your roof can collect each year.
This helps plan your tank size, filtration system, and water usage goals efficiently.