Rainwater Harvest Calculator
Calculate how much rainwater you can collect from your roof for sustainable water usage.
Selected: 85%
Rainwater Harvest Results
Potential Water Collection:
In Gallons:
In Liters:
Number of 55-Gallon Barrels:
About Our Rainwater Harvest Calculator
Our Rainwater Harvest Calculator is an essential tool for anyone interested in water conservation, sustainable living, or reducing water bills. It helps you estimate how much rainwater you can potentially collect from your roof based on its size and local rainfall patterns.
Why Harvest Rainwater?
Rainwater harvesting is an ancient practice that has become increasingly relevant in modern times. Collecting rainwater reduces demand on municipal water supplies, lowers water bills, decreases stormwater runoff, and provides a sustainable water source for many household and garden needs. It's an environmentally friendly way to conserve one of our most precious resources.
The Rainwater Collection Formula
The formula for calculating potential rainwater collection volume is:
Volume = Roof Area × Rainfall Depth × Collection Efficiency
Where:
- Roof Area is the footprint of your roof in square feet
- Rainfall Depth is the amount of rainfall in inches
- Collection Efficiency accounts for losses due to evaporation, splash, and system inefficiencies (typically 80-85%)
- The result is converted to gallons using the factor: 0.623 gallons per square foot per inch of rain
Key Features:
- Calculate potential rainwater collection from any roof size
- Adjust for different rainfall amounts based on your local climate
- Account for system efficiency with an adjustable efficiency slider
- View results in multiple units (cubic feet, gallons, and liters)
- Estimate how many standard rain barrels your collection could fill
How to Use:
- Enter your roof's catchment area in square feet (length × width for simple roofs)
- Input the average rainfall for your area in inches (monthly or annual)
- Adjust the collection efficiency if needed (default is 85%, which is typical)
- Click "Calculate Rainwater Harvest" to see your potential collection volume
Finding Your Roof Area:
For simple roofs, measure the length and width of your home's footprint (the area covered by your roof). For complex roof designs, break the roof into rectangular sections, calculate the area of each section, and add them together. Remember that the calculator uses the horizontal projection of your roof (its footprint), not the actual surface area of sloped roofs.
Finding Your Local Rainfall:
To find the average rainfall for your area:
- Check your local weather service website
- Search online for "[your city] average rainfall"
- Use weather apps that provide historical precipitation data
- Contact your local agricultural extension office
You can calculate based on monthly averages for seasonal planning or annual totals for overall system sizing.
Common Uses for Harvested Rainwater:
Garden Irrigation: Using rainwater for plants provides chemical-free nourishment.
Lawn Watering: Reduce water bills by using stored rainwater during dry periods.
Car Washing: Rainwater is soft and free of minerals that can spot vehicles.
Toilet Flushing: With proper filtration and system setup, rainwater can be used indoors.
Emergency Water Supply: With appropriate treatment, stored rainwater can serve as backup water.
Whether you're setting up a simple rain barrel or designing a comprehensive rainwater harvesting system, our calculator helps you understand your collection potential and plan accordingly. Start calculating your rainwater harvesting capacity today!
Frequently Asked Questions
What affects collection efficiency in a rainwater harvesting system?
Collection efficiency is influenced by several factors: roof material (smoother surfaces like metal have higher efficiency than rough surfaces like asphalt shingles), first-flush diverters (which reduce efficiency but improve water quality by diverting initial dirty runoff), evaporation and splash-out, gutter design and maintenance, and filtration systems. Most residential systems operate at 80-85% efficiency under optimal conditions.
Do I need to account for my roof pitch in calculations?
No, our calculator uses the horizontal projection (footprint) of your roof, not the actual surface area. While a steeper roof has more surface area, the amount of rain that falls on it depends on the horizontal area it covers. This is why you should use your home's footprint dimensions rather than measuring along the slope of your roof.
Is rainwater safe to use for all purposes?
Without proper treatment, harvested rainwater is generally considered non-potable (not for drinking). It's safe for outdoor uses like irrigation, car washing, and water features. For indoor non-potable uses like toilet flushing, basic filtration is recommended. For potable (drinking) uses, comprehensive filtration, disinfection, and regular water quality testing are necessary, and local regulations may apply. Always check your local health department regulations regarding rainwater use.
How much storage capacity do I need for my rainwater system?
The ideal storage capacity depends on your rainfall patterns, water usage needs, and available space. As a general guideline, you'll want enough capacity to store water from a typical heavy rainfall event in your area. For small residential systems, multiple 55-gallon barrels are common. For larger systems, storage tanks ranging from 500 to 5,000+ gallons might be appropriate. Consider both your collection potential and how you plan to use the water when determining storage needs.