Rainwater Harvesting Calculator
Water you can collect from your roof.
Enter your values
Enter the roof catchment area in sq ft.
Enter the rainfall in inches.
Enter the collection efficiency in %.
Enter the annual rainfall in inches.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Water you can collect from your roof. |
|---|---|
| Category | Home & DIY |
| Inputs needed | Roof catchment area, Rainfall, Collection efficiency, Annual rainfall |
| Main output | Gallons per rainfall event |
| Formula | Gallons = area × inches × 0.623 × efficiency |
| Cost | Free — no sign-up, no download |
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%).
Inputs explained
- Roof catchment area(sq ft)
- Enter the roof catchment area in sq ft.
- Rainfall(inches)
- Enter the rainfall in inches.
- Collection efficiency(%)
- Enter the collection efficiency in %.
- Annual rainfall(inches)
- Enter the annual rainfall in inches.
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.
What do I need to enter into the Rainwater Harvesting Calculator?
Just 4 values: roof catchment area, rainfall, collection efficiency, annual rainfall. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Rainwater Harvesting Calculator work out the answer?
One inch of rain on 1000 square feet of roof yields about 600 gallons, before collection losses. It applies the formula Gallons = area × inches × 0.623 × efficiency and shows the working so you can check each step by hand.
Is the Rainwater Harvesting Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.