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🌧️ Rain Harvesting Calculator
Estimate how much rainwater you can collect from your roof based on area, annual rainfall, and collection efficiency.
How Rain Harvesting Is Calculated
The formula is: Gallons = Roof Area (sq ft) × Rainfall (inches) × 0.623 × Efficiency. The 0.623 factor converts 1 inch of rain on 1 square foot into gallons. Typical collection efficiency is 75–90%, accounting for losses from evaporation, splash, first-flush diverters, and gutter overflow.
Storage Sizing Tips
A standard rain barrel holds 55 gallons, and an IBC tote holds 275 gallons. For most households, storing 1–2 months of collection is a good target. Harvested rainwater is great for irrigation, toilet flushing, and laundry. Check local regulations — some areas restrict or incentivize rainwater collection.
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