Roof Rainwater Collection Calculator
A roof is a large rainwater catchment, and a simple calculation tells you how much it can yield. The harvestable volume is the roof's horizontal footprint area times the rainfall depth times a runoff coefficient for real-world losses. In metric units the arithmetic is clean: one millimetre of rain on one square metre is one litre. This calculator returns the collectable volume in litres, cubic metres and US gallons. Enter your roof footprint, local rainfall and a runoff coefficient; all are user-editable inputs.
Roof collection formula
Collectable litres = area (m2) * rainfall (mm) * runoff coefficient
Cubic metres = litres / 1,000
US gallons = litres / 3.78541
Average daily yield = annual litres / 365
The unit identity 1 mm * 1 m2 = 1 litre makes the litre figure direct. The runoff coefficient removes losses. Daily yield assumes the rainfall figure is an annual total.
Designing for the yield
- Use the horizontal footprint, not the sloped roof area.
- A first-flush diverter improves quality but slightly lowers the coefficient.
- Pair this yield with tank sizing so storage matches both supply and demand.
- Use long-term local rainfall, not a single wet or dry year.
Roof rainwater collection: frequently asked questions
How much rainwater can a roof collect?
The harvestable volume is the roof's catchment area times the rainfall depth times a runoff coefficient that accounts for losses. In consistent metric units, area in square metres times rainfall in millimetres gives litres directly (1 mm over 1 m2 is 1 litre). This calculator applies your runoff coefficient and reports the annual yield.
What is a runoff coefficient?
It is the fraction of rain falling on the roof that actually reaches the tank, after losses to evaporation, splashing, first-flush diversion and the roof material's absorption. Smooth metal roofs have high coefficients (often 0.8 to 0.9); rougher surfaces are lower. It is a user-editable input here.
Should I use the roof footprint or the sloped area?
Use the horizontal projected (footprint) area, not the sloped surface area. Rain falls vertically, so the catchment is the plan area the roof covers, regardless of pitch. Measure the building footprint under the roof.
Where do I get rainfall figures?
Use long-term annual rainfall for your location from your national meteorological agency, such as the US National Weather Service. Enter annual rainfall for a yearly yield, or a single storm's depth to size first-flush and overflow.
Official sources
- U.S. Environmental Protection Agency: Soak Up the Rain: Rain Barrels.
- U.S. National Oceanic and Atmospheric Administration: National Weather Service rainfall data.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.