Rain Barrel Fill Calculator
See how many gallons your roof sheds in a rainfall and how many rain barrels that fills.
How it works
gallons = roof area (sq ft) × rainfall (in) × 0.623 × efficiency
One inch of rain falling on one square foot of roof yields about 0.623 gallons — a surprisingly large number that catches most people off guard. Use the flat footprint of the roof draining to your downspout, not the sloped area, since rain falls vertically. Real collection loses a bit to the first-flush diverter, splash and evaporation, so 75–90% efficiency is realistic. The headline is how quickly a modest storm overwhelms a single barrel.
Worked example
A 1,000 sq ft roof section in a 1-inch rain sheds 1,000 × 1 × 0.623 = 623 gallons, and at 85% efficiency about 530 gallons reach the barrel. That fills a 55-gallon barrel roughly 9.6 times over — it takes only about 0.10 inch of rain to top one barrel.
Frequently asked questions
Should I use the sloped roof area or the footprint?
Use the flat footprint (length × width of the area draining to that downspout). Rain falls vertically, so the collecting area is the horizontal projection — roof pitch does not change how many gallons land on it.
Is rain-barrel water safe for a vegetable garden?
It’s fine for watering soil around most plants, but roof runoff can carry bird droppings, shingle grit and debris. Use a first-flush diverter, keep it covered to block mosquitoes and debris, and water the soil rather than edible leaves directly.
How do I handle the overflow?
Fit an overflow diverter that routes excess away from the foundation, or chain several barrels together so they fill in series. As the example shows, one barrel fills in a fraction of an inch of rain, so overflow is the normal case, not the exception.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.