About this tool
The Roof Rainwater Runoff Calculator estimates how much water a roof or any catchment surface will shed during a rain event. It is the number you need to size a rain barrel or cistern, plan gutters and downspouts, or gauge how much water a rainwater-harvesting system can capture from a single storm.
The calculation follows the rational method: runoff volume = catchment area × rainfall depth × runoff coefficient. In US units, gallons = area(ft²) × rainfall(in) × 0.623377 × coefficient, where 0.623377 converts one inch of rain over one square foot (144 in³) into US gallons. In metric, one millimeter of rain over one square meter equals exactly one liter, so liters = area(m²) × rainfall(mm) × coefficient. The runoff coefficient defaults to 0.90, typical of hard, low-absorption roofs such as metal or asphalt shingle.
Everything is computed locally in your browser. Enter the roof footprint area (ft² or m²) — use the flat plan area, not the sloped surface, since a slope does not change how much rain lands on the footprint — the rainfall depth (in or mm), and the runoff coefficient (0–1). Results are shown in US gallons, liters, and cubic meters.