Rain Barrel Size Calculator
Rainwater collected from your roof for barrel sizing.
Enter your values
Enter the roof catchment area in sq ft.
Enter the rainfall amount in in.
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.
- Water collected
- 935 gal
- 55-gallon barrels needed
- 17
- Per 1 inch of rain
- 935 gal
Quick answer
Collected rainwater is roof catchment area times rainfall depth times 0.623 gallons per sq ft per inch of rain.
gallons = roof area (sq ft) x rainfall (in) x 0.623
At a glance
| What it does | Rainwater collected from your roof for barrel sizing. |
|---|---|
| Category | Home & DIY |
| Inputs needed | Roof catchment area, Rainfall amount |
| Main output | Water collected |
| Formula | gallons = roof area (sq ft) x rainfall (in) x 0.623 |
| Cost | Free — no sign-up, no download |
How to use the Rain Barrel Size Calculator
- 1Enter the roof catchment area feeding into your gutter/downspout system.
- 2Enter the rainfall amount for a typical storm in your area.
- 3See total gallons collected and how many standard 55-gallon rain barrels you'd need.
Inputs explained
- Roof catchment area(sq ft)
- Enter the roof catchment area in sq ft.
- Rainfall amount(in)
- Enter the rainfall amount in in.
Worked example
Using the values the calculator loads with:
Inputs
- Roof catchment area1500 sq ft
- Rainfall amount1 in
Results
- Water collected935 gal
- 55-gallon barrels needed17
- Per 1 inch of rain935 gal
Frequently asked questions
Why 0.623 gallons per sq ft per inch?
It's the standard conversion factor: 1 inch of rain over 1 sq ft equals 0.623 gallons of water.
Do I need an overflow system?
Yes, always include an overflow diverter since a single storm can produce far more water than a barrel can hold.
What do I need to enter into the Rain Barrel Size Calculator?
Just 2 values: roof catchment area, rainfall amount. 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 Rain Barrel Size Calculator work out the answer?
Collected rainwater is roof catchment area times rainfall depth times 0.623 gallons per sq ft per inch of rain. It applies the formula gallons = roof area (sq ft) x rainfall (in) x 0.623 and shows the working so you can check each step by hand.
Is the Rain Barrel Size 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.