Rain Barrel Calculator
Water collected from roof and rainfall
Rainwater Harvesting
Water collected
0
※ 1 inch of rain on 1 sq ft ≈ 0.623 gallons (1mm on 1㎡ = 1 L). Efficiency 80-90% accounts for splash, evaporation and first-flush loss. Use the roof footprint (plan area), not the sloped area.
Details
⚠️ Reference estimate based on typical values. Actual amounts vary with crop, soil, climate and product. Verify with local guidance and product labels. calcstool.com assumes no liability for results.
FAQ
How much rain can I collect?
About 0.623 gallons per sq ft per inch of rain (1 L per ㎡ per mm). A 1000 sq ft roof in 1 inch of rain yields ~530 gallons at 85% efficiency.
Why use the footprint?
Use the plan (footprint) area of the roof, not the sloped surface — the footprint is what catches the rain.
What efficiency?
80-90% is typical, accounting for splash, evaporation and first-flush diversion.
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