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Rain Barrel Overflow Time Calculator
A rain barrel can fill surprisingly quickly because one inch of rain on one square foot of roof produces about 0.623 gallons before losses. Overflow timing depends on rainfall intensity, connected roof area, available empty capacity, and collection efficiency. This calculator shows when the overflow path must begin carrying water rather than treating barrel volume as the full storm capacity.
Planning estimate only. Check measurements and real-world constraints before buying materials or making a commitment.
Calculate your scenario
Change any input. Results update immediately.
Your results
Runoff capture rate
158.9 gal/hour
Average inflow at the entered rain intensity.
Time until overflow
17 minutes
Assumes no water is drawn during the storm.
Estimated overflow volume
272.7 gal
Water needing a safe overflow route.
How the calculation works
The calculator applies this relationship to the inputs above. Keep every measurement in the unit shown.
Worked example
Use this example to check the calculator by hand before relying on a result.
Assumptions behind the result
- • Rainfall intensity is averaged over the event.
- • Roof area drains to one collection point.
- • Barrel capacity means empty space.
- • No water is used during rainfall.
- • Overflow is routed away from foundations.
Mistakes that change the answer
- • Using total roof area when only one downspout is connected.
- • Using label capacity for a partly full barrel.
- • Failing to provide a safe overflow path.