Rainwater Barrel Calculator

Rain barrels and cisterns collect runoff from roofs and other impervious surfaces. Yield approximates catchment area multiplied by rainfall depth multiplied by a runoff coefficient that accounts for losses. The coefficient is a fraction between 0 and 1 — perfect capture would be 1.0, but real systems lose water to first flush, splash, clogged gutters, and overflow when the barrel fills.

US mode uses roof area in square feet and rainfall in inches. One inch of rain on 1,000 sq ft of roof produces about 623 gallons if every drop were captured — a useful sanity check before you trust a smaller coefficient. Metric mode uses square meters and millimeters of rain, then converts the result to gallons and liters for comparison with common barrel sizes.

Smooth metal and asphalt shingle roofs often use coefficients around 0.85–0.95. Flat gravel or green roofs may be lower. First-flush diverters intentionally waste the dirtiest initial runoff to keep sediment out of storage — lower your coefficient if you divert that water away from the barrel.

Enter rainfall for one storm when sizing overflow paths, or use monthly totals when estimating seasonal irrigation supply. A 55-gallon barrel fills quickly on a large roof — plan overflow hose routing away from foundations. Multiple barrels in series increase storage without changing per-storm capture math.

Roof runoff is not potable without treatment — use it for gardens, ornamentals, and outdoor cleaning unless local code explicitly allows more. Check homeowners association rules and municipal rainwater harvesting ordinances before installing permanent plumbing.

Barrel capacity is a storage limit, not a yield limit. This calculator estimates how much water arrives; you may capture only until the tank fills and the rest overflows. Match barrel count to typical storm size or accept regular overflow during heavy rain. Screen inlets keep mosquitoes and debris out of stored water between storms.

Roof area should reflect only the surfaces draining into your downspout — not the entire footprint of a multi-wing house unless gutters combine all flows into one barrel line. Measure the sloped surface projected to plan area, or use footprint if you lack pitch measurements.

Worked example with default inputs: an 800 sq ft roof with 1 inch of rain and a 0.85 runoff coefficient yields about 424 gallons (~1,605 liters). That exceeds a single 55-gallon barrel — expect overflow unless you chain tanks or divert the first flush. A half-inch storm would deliver roughly half that volume.