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Worked Examples
- 1.Roof area: 1,500 sq ft
- 2.Convert rainfall to feet: 36 / 12 = 3 ft
- 3.Volume in cubic feet: 1,500 x 3 = 4,500 cu ft
- 4.Convert to gallons: 4,500 x 7.48052 = 33,662 gallons (raw)
- 5.Apply 80% efficiency: 33,662 x 0.80 = 26,930 gallons/year
- 6.Daily average: 26,930 / 365 = 73.8 gallons/day
- 7.Recommended tank: 50 x 14 = 700 gallons
A 1,500 sq ft roof collecting 36 inches of rain at 80% efficiency yields about 26,930 gallons per year. A 700-gallon tank covers 14 days of 50 gallons/day usage.
How Much Rainwater Can You Collect?
Formula
Rainwater harvesting captures precipitation from your roof and stores it for later use such as garden irrigation, toilet flushing, or laundry. Even a modest roof in a moderate rainfall area can collect thousands of gallons per year. The basic formula converts roof area and rainfall depth into volume: every 1 inch of rain on 1 square foot of roof produces about 0.623 gallons.
The efficiency factor accounts for real-world losses from evaporation, splash, first-flush diverters, and gutter overflow. Metal roofs achieve 90-95% efficiency, while asphalt shingles typically capture 75-85%. Tank sizing is usually based on 2 weeks of expected usage to bridge dry spells, though local dry-season length should be considered for more precise sizing.
Common use cases:
- Garden and landscape irrigation planning
- Sizing rainwater storage tanks
- Reducing municipal water bills
- Sustainable home water management
Frequently Asked Questions
Maria Gonzalez
Registered Dietitian, RD, MPH
Maria is a Registered Dietitian with a Master's in Public Health. She focuses on evidence-based nutrition assessment tools including BMI, calorie calculations, and body composition analysis.
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