Rain Barrel Calculator
Calculate rainwater harvest from your roof per storm or per year.
Enter roof area, rainfall amount, and barrel size to plan your collection system.
Rainwater harvesting captures precipitation from your roof and stores it for garden use. A rain barrel system can significantly reduce municipal water usage for irrigation, saving money and conserving resources.
The collection formula:
Gallons Collected = Roof Area (sq ft) × Rainfall (inches) × 0.623 × Collection Efficiency
The factor 0.623 converts square feet and inches of rainfall into gallons. One inch of rain falling on one square foot of roof produces 0.623 gallons (2.36 liters) of water.
Collection efficiency: Not all rainwater that hits the roof makes it into the barrel. Typical efficiency factors:
- Smooth metal roofing: 90-95% efficiency
- Asphalt shingles: 80-85% efficiency
- Tile or slate: 75-80% efficiency
- Flat roofs: 70-75% efficiency (water pools and evaporates)
Additional losses come from gutter spillover, first-flush diverters (which discard the first few gallons to remove debris), and splash-out from the collection point.
How much water a garden needs: A typical vegetable garden needs approximately 1 inch of water per week during the growing season. This equals about 0.62 gallons per square foot per week. A 200 sq ft garden needs approximately 124 gallons per week.
Roof collection area: Measure your roof footprint (the ground area it covers), not the actual roof surface area. A house with a 30×40 foot footprint has a 1,200 square foot collection area even if the roof is pitched. If you only connect one downspout, estimate the fraction of roof area that drains to that downspout.
Rainfall data: Average monthly rainfall varies dramatically by location. Use your local weather data for accurate estimates. For example, Seattle averages about 3.5 inches per month, while Phoenix averages about 0.7 inches per month.
Storage sizing: Standard rain barrels are 55 gallons (208 liters). Larger cisterns range from 100 to 10,000 gallons. A good rule is to have enough storage to capture the water from one average rain event in your area. Connecting multiple barrels in series increases capacity.
Legal considerations: Most US states now allow rainwater collection, but some have regulations on system size or usage. Check your local laws before installing a system.