Rain Barrel Collection Calculator
Estimate how much rainwater you can collect per year from your roof based on roof area and annual rainfall.
Supports gallons and liters.
Annual Collection
Rainwater collection depends on roof area, annual rainfall, and a runoff coefficient.
Collection = Roof Area × Annual Rainfall × Runoff Coefficient
Runoff coefficient: ~0.80 (80%) for typical shingled roofs — accounts for evaporation, absorption, and splash.
Unit conversions:
- Imperial: roof in sq ft, rainfall in inches → gallons
Gallons = Sq Ft × Inches × 0.623
- Metric: roof in sq m, rainfall in mm → liters
Liters = Sq M × mm × 1
Then multiply by 0.80 for the runoff coefficient.
US average rainfall: ~30–60 inches/year (760–1,520 mm)
- Seattle: 37 in · New York: 50 in · Miami: 62 in · Phoenix: 8 in
Standard rain barrel: 55 gallons (208 L)