Rainwater Harvesting Calculator
Estimate how much rainwater your roof can collect and size your storage tank for off-grid water supply.
New to off-grid? Start with our 3-question path finder for a personalized off-grid guide.
Calculation Assumptions
- Monthly collection = Roof area (m²) × Rainfall (mm/month) × Collection efficiency. 1 mm of rain on 1 m² yields exactly 1 liter.
- Collection efficiency accounts for losses from evaporation, first-flush diversion, gutter overflow, and debris. 80% is a reasonable default for a metal roof with clean gutters.
- Recommended tank size = daily usage × reserve days. Size for your driest month, not the annual average.
- Rainfall figures are monthly averages — actual collection will vary seasonally. Always assess your dry-season minimums when sizing a primary supply tank.
Harvest Summary
Updates live as you adjust inputs.
Monthly Collection
793gal/mo
Daily average: 26.4 gal/day
At 75 mm/month · 80% efficiency
Supply vs. Demand
20.0days/month
⚠Covers 67% of monthly usage — supplement needed
Recommended Tank Size
1,189gal
30-day reserve at 39.6 gal/day usage