Rainwater Harvesting Calculator
Water you can collect from your roof.
Enter your values
Results update instantly as you type — no submit needed.
Results
- Gallons per rainfall event
- 953
- Annual harvest
- 24,147 gallons
- Litres per event
- 3,608
- Barrels needed (55 gal)
- 18
- Monthly average
- 2,012 gallons
Quick answer
One inch of rain on 1000 square feet of roof yields about 600 gallons, before collection losses.
Gallons = area × inches × 0.623 × efficiency
How to use the Rainwater Harvesting Calculator
- 1Measure your roof footprint, not the sloped area.
- 2Enter the rainfall depth in inches.
- 3Set a realistic collection efficiency (80–90%).
Frequently asked questions
Is roof pitch relevant?
No — the horizontal footprint determines catchment.
Is rainwater safe to drink?
Not without treatment; use it for irrigation unless properly filtered.