Rainwater Harvesting Calculator
Water you can collect from your roof.
Enter your values
ⓘ Live results
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%).
Worked example
Using the values the calculator loads with:
Inputs
- Roof catchment area1200 sq ft
- Rainfall1.5 inches
- Collection efficiency85 %
- Annual rainfall38 inches
Results
- Gallons per rainfall event953
- Annual harvest24,147 gallons
- Litres per event3,608
- Barrels needed (55 gal)18
- Monthly average2,012 gallons
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.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.