Rainwater Harvesting Calculator
See how many gallons of rain your roof (or any collector surface) can catch.
What you can catch
Water collected
55-gallon barrel fills with
Gallons = area (sq ft) × rainfall (in) × 0.623
How it’s figured
- One inch of rain over one square foot equals about 0.623 gallons, so gallons = area × inches × 0.623.
- Inches to fill a 55-gallon barrel = 55 ÷ (area × 0.623).
- This is the theoretical catch; real systems lose some water to splashing, evaporation, and the first-flush diverter.