Rain Barrel Calculator
Estimate how much rainwater a roof collects from its catchment area and the rainfall, with a collection efficiency, in gallons, litres, and barrels. Calculates in your browser.
Water harvested
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Show the math
Every inch of rain on a square foot of roof yields about 0.623 gallons:
gallons = roof area × rainfall × 0.623 × efficiency
Roof area is the footprint, not the sloped surface, rain falls vertically. A small efficiency loss covers spillage and first-flush diversion. Check local rules on rainwater collection. All math runs locally in your browser.
What this does
A rain barrel calculator estimates how much rainwater a roof collects from its catchment area and the rainfall, with a collection efficiency, in gallons, litres, and barrels.
How to use it
- Enter the roof footprint.
- Enter the rainfall.
- Set the collection efficiency.
- Read the gallons and barrels collected.
How it works
Use the roof footprint, since rain falls vertically. An 85–90% efficiency allows for splashing, evaporation, and first-flush losses.
gallons = area × rain (in) × 0.623 × efficiency
Understanding your result
Even modest roofs collect surprising amounts, which is why a single barrel often overflows in one good storm. The efficiency factor allows for splashing, evaporation, and first-flush losses, so size your storage and overflow for the real yield rather than the raw catchment figure.
Example
An inch of rain on a 1,000 sq ft roof yields around 560 gallons before losses.
Sources & methodology
Last updated .
Frequently asked questions
What roof area do I use?
Use the footprint (the area the roof covers seen from above) not the sloped surface, because rain falls vertically.
Why an efficiency factor?
Some water is lost to splashing, evaporation, and first-flush diversion, so 85–90% collection is a realistic estimate.
How much does a roof collect?
About 0.623 gallons per square foot per inch of rain, so an inch on a 1,000 sq ft roof is around 560 gallons before losses.
Is anything stored?
No. Everything is calculated locally in your browser.
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