How Do You Calculate Rainwater Collection on a Roof?

Rainwater harvesting is one of the simplest ways to save water. Whether you want to fill a storage tank, feed a garden, or just understand how much water your roof can collect, the calculation is very easy.

In this guide, we’ll break it down into simple steps and show 2–3 examples so you can calculate it for any building.

1. What You Need for the Calculation

To find out how much rainwater your roof can collect, you only need three things:

1. Roof Area (in square feet or square meters)
2. Rainfall Amount (in inches or millimeters)
3. Runoff Coefficient (usually between 0.8 to 0.95)

That’s it!

2. Basic Formula for Rainwater Collection

Formula

$\text{Rainwater Collected}=\text{Roof Area}\times \text{Rainfall}\times \text{Runoff Coefficient}$

3. Important Notes

• Runoff coefficient adjusts for losses (evaporation, splash, absorption).
  • Metal or concrete roof: 0.9–0.95
  • Tile or shingle roof: 0.8–0.9
• If you measure rainfall in inches, you need a conversion:

1 inch of rainfall on 1 sq. foot = 0.623 gallons

Formula

$\text{Gallons}=\text{Roof Area}\times \text{Rainfall (inches)}\times 0.623\times C$

(where C = runoff coefficient)

If you don’t want to calculate everything manually, you can simply use our Roof Water Collection Calculator. Just enter your roof area, rainfall, and runoff coefficient, and the tool will instantly give you accurate results. It’s quick, easy, and saves you time. You can try it here:

Examples

Example 1: Small house, 1000 sq. ft roof, 1 inch rainfall

• Roof Area = 1000 sq. ft
• Rainfall = 1 inch
• Runoff coefficient = 0.9 (for metal roof)

Step-by-step calculation

$1000\times 1\times 0.623\times 0.9=560.7$

So, 1 inch of rain gives you about 560 gallons of water.

Example 2: 150 sq. meter roof, 20 mm rainfall

Let’s use metric units this time.

Metric Formula

$\text{Liters}=\text{Roof Area (m²)}\times \text{Rainfall (mm)}\times C$

Now plug in values:

• Roof Area = 150 m²
• Rainfall = 20 mm
• Coefficient (tile roof) = 0.85

$\text{Liters}=150\times 20\times 0.85$

$\text{Liters}=2550\text{ liters}$

So, this roof collects about 2550 liters from a 20 mm rain.

Example 3: 2000 sq ft roof, 2 inches rainfall, shingle roof

• Area = 2000 sq ft
• Rainfall = 2 inches
• Coefficient = 0.85

$\text{Gallons}=2000\times 2\times 0.623\times 0.85$

$\text{Gallons}=2116.2$

So, this home can collect about 2100 gallons from a 2-inch rain.

Quick Reference Table

Final Tips

• Keep gutters clean for maximum collection.
• Use a first-flush system to remove dust and debris.
• Bigger tank ≠ always better — calculate your needs first.
• Roof angle does not change volume — only roof area matters.

Conclusion

Calculating rainwater collection is straightforward once you know the formula. Just multiply your roof area by the rainfall and the runoff coefficient, and you’ll have a solid estimate of how much water you can store.