Rainwater Barrel Calculator
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Gallons or liters from roof catchment × rainfall × runoff factor.
Catchment
Water captured
424 gal
1,605 L
- Liters
- 1,605
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Rainwater yield
Rain barrels collect runoff from roofs. Yield ≈ catchment area × rainfall depth × runoff coefficient. The coefficient accounts for losses (first flush, splash, dry soil on flat areas) — roof surfaces often use 0.8–0.95.
Enter rainfall for one storm or monthly total depending on your planning. One inch on 1,000 sq ft roof ≈ 623 gallons at 100% capture; real systems capture less.
Example: 800 sq ft roof, 1 in rain, 0.85 coefficient → ~424 gallons (~1,605 L). Check local rainwater harvesting rules.
Common questions
What runoff coefficient should I use?
Smooth metal or asphalt shingle roofs often 0.85–0.95. Use lower values if you expect significant first-flush diversion.
Can I drink this water?
Roof runoff is not potable without treatment. Use for irrigation unless your local code allows otherwise.