Rainwater Harvesting Calculator
Calculate rainwater collection potential, storage requirements, and cost savings for your roof
Calculate Rainwater Harvesting Potential
Measure the length of your roof
Measure the width of your roof
Typical: 15-45°, Flat roof = 0°
Highest collection efficiency, smooth surface
Check local weather data
Check your water utility bill
Rainwater Harvesting Results
Effective Roof Area: 1385.6 ft²
Collection Efficiency: 95.0% (based on Metal (Galvanized Steel))
Recommended Storage Tank: 4,044.286 gal (45-day supply)
First Flush Volume: 34.5 gal (should be diverted for water quality)
Financial Benefits
Annual Savings
$114.81
Monthly Savings
$9.57
System Cost Est.
$5,557.611
Payback Period
48.4 years
Collection Analysis
Example Calculation
Typical Residential Home
Location: Seattle, WA (40 inches annual rainfall)
Roof: 40 ft × 30 ft with 30° pitch
Material: Metal roof (95% efficiency)
Effective Area: 1,385 ft²
Results
Annual Collection: 32,615 gallons
Monthly Average: 2,718 gallons
Daily Average: 89 gallons
Annual Savings: ~$114 (at $3.50 per 1,000 gallons)
System Cost: ~$4,000-$6,000
Payback: 35-52 years (or less with higher water rates)
Roof Material Efficiency
Essential Components
Collection System
Gutters, downspouts, leaf guards
First Flush Diverter
Diverts initial dirty water
Storage Tank
Cistern or rain barrels
Distribution System
Pumps, pipes, controls
Water Harvesting Tips
Metal roofs provide best collection efficiency (95%)
Install first flush diverter for cleaner water
Size storage for 30-60 days of average collection
Check local regulations before installation
Use harvested water for irrigation and non-potable uses
Understanding Rainwater Harvesting
What is Rainwater Harvesting?
Rainwater harvesting is the collection and storage of rainwater from rooftops or other catchment surfaces for later use. It's an ancient practice gaining renewed importance as communities face water scarcity and seek sustainable water management solutions.
Why Harvest Rainwater?
- •Reduces dependence on municipal water supply
- •Lowers water bills and utility costs
- •Provides water during droughts or restrictions
- •Reduces stormwater runoff and erosion
- •Rainwater is naturally soft and chlorine-free
Calculation Formula
Collection = Roof Area × Rainfall × Runoff Coefficient
(Imperial: ft² × inches × 0.623 × coefficient = gallons)
(Metric: m² × mm ÷ 1000 × 1000 × coefficient = liters)
- Roof Area: Effective catchment surface area
- Rainfall: Annual or monthly precipitation
- Runoff Coefficient: Material efficiency (0.40-0.95)
- 0.623: Conversion factor for Imperial units
Note: Actual collection may vary based on rainfall patterns, evaporation, and system losses. This calculator provides theoretical maximum collection.
Benefits of Rainwater Harvesting
Water Conservation
Reduce demand on municipal water supply and preserve freshwater resources for future generations.
Cost Savings
Lower water bills, reduced stormwater fees, and potential rebates from local utilities.
Environmental Impact
Reduce runoff, prevent erosion, decrease flooding, and support sustainable living practices.