The garden workbench
Estimate roof runoff, retained water and overflow for one rainfall event.
Measure the horizontal footprint from ground-level plans, not the sloped roof surface. Include only the roof draining to the connected downspout. The 80% efficiency value is an editable example allowing for losses, not a rating for your system.
A large collection surface can fill a small barrel quickly. In the example, 500 square feet receiving one inch of rain has about 312 US gallons of theoretical runoff. At 80% collection efficiency, about 249 gallons reach storage. Two connected 55-gallon barrels already holding 20 gallons have room for only 90 more; roughly 159 gallons would overflow.
Check where that overflow can go, keep the container covered and securely supported, and follow local installation and water-use guidance. Roof runoff is non-potable. A volume result does not establish water quality or suitability for edible crops.
US gallons = square feet × inches of rain × 144 ÷ 231. Liters = square meters × millimeters of rain. Multiply by collection efficiency, then cap the retained amount at unused storage capacity. The remaining inflow is overflow. The calculation assumes gutters can deliver the water; actual capture also depends on storm intensity, bypasses, maintenance and system design.
University of Nebraska–Lincoln: Rainwater Harvesting with Rain Barrels explains roof catchment and storage limitations. This tool uses the exact unit conversion rather than the rounded 0.623 factor.
Use these guides to check the growing decisions behind your inputs.
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Understand low-pressure rain-barrel irrigation, safe placement, non-potable water and the checks needed before connecting a timer or drip line.
Read the guideSeasonal gardening · 6 min read
Compare infiltration and storage, map roof runoff and overflow, and identify the site questions to resolve before installing a rain garden or barrel.
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