Rain Barrel Calculator
Even a modest roof sheds a surprising amount of water. Enter the area that drains to your downspout and a rainfall amount to see how much you can collect, and how many barrels it takes to store it.
View formula
How to use the Rain Barrel Calculator
- Measure the footprint of the roof section that drains to the downspout (length × width as seen from above).
- Enter a rainfall amount, a single storm or a monthly average.
- Keep 75 to 90% efficiency for typical gutters.
- Enter your barrel size and press Calculate.
Rainwater collection formula
Every inch of rain on 1,000 sq ft of roof produces about 623 gallons before losses, far more than one barrel can hold.
Worked example
A 1,000 sq ft roof section, a 1 inch storm and 80% efficiency:
- 1,000 × 1 × 0.623 × 0.8 = 499 gallons
- 55-gallon barrels to store it all: 10
- One barrel fills from just 0.11 in of rain.
Planning your system
- Use the roof’s horizontal footprint, not the sloped surface, the pitch doesn’t change how much rain lands on it.
- Link barrels together or use an overflow hose that directs extra water away from foundations.
- Fit a screen or lid to keep out debris and mosquitoes, and a first-flush diverter for cleaner water.
- Rules on rainwater collection vary by location, check local regulations before installing large tanks.
FAQ
Rain Barrel Calculator: frequently asked questions
How much water comes off a roof in 1 inch of rain?
About 0.623 gallons per square foot of roof footprint, 623 gallons per 1,000 sq ft before losses.
How fast does a rain barrel fill?
Quickly. A 55-gallon barrel on a 1,000 sq ft roof section fills with roughly a tenth of an inch of rain at 80% efficiency.
Is rainwater safe for vegetables?
Rainwater is widely used for gardens. Many gardeners apply it to the soil rather than leaves of edible crops and avoid collecting from treated-wood or old asphalt roofs where possible.
What efficiency should I use?
75 to 90% is typical. Use the lower end for older gutters, gravel roofs or if you divert the first flush of each storm.
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