Rainwater Harvesting Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/4/2026
The harvestable rainwater volume is calculated with roof surface area (m²) × annual rainfall (mm/year) × yield coefficient. For a 100 m² roof with 800 mm of rain per year and an 85% yield, the harvestable volume is 68,000 liters per year.
Explanation
One millimeter of rain falling on one square meter represents exactly one liter of water: it's this direct equivalence that lets you calculate the theoretically available rainwater volume by simply multiplying the roof surface area by the local annual rainfall. In practice, no roof harvests all of that water: some is lost to evaporation, absorbed by the roofing material, or diverted by a first-flush system (which discards the dirtiest water at the start of each downpour). This yield coefficient mainly varies by roofing material: a standard tile roof typically offers a yield of about 80%, slate about 85%, steel sheet or zinc between 90 and 95% (the best performance, thanks to a smooth, non-porous surface), while a gravel-topped flat roof often drops to between 60 and 80% due to greater water retention. The surface area to use is the roof's projected footprint (its horizontal extent seen from above), not the sloped real surface area — unlike the calculation used for our sloped roof surface area calculator, which accounts for the pitch. For the instantaneous drainage flow rate this same roof needs to handle during a storm (a different question from the annual harvestable volume), see our roof rainwater flow rate calculator.
Example: a 100 m² roof under 800 mm of annual rainfall
Inputs
Surface area: 100 m². Rainfall: 800 mm/year. Yield: 85%.
Calculation
Harvestable volume = 100 × 800 × 0.85 = 68,000 liters per year.
Result
This roof can theoretically harvest 68,000 liters of rainwater per year.
Frequently asked questions
Where do I find my region's annual rainfall?
National weather services typically publish average annual rainfall figures by region or city, calculated over several decades. These averages vary significantly across France, from about 500-600 mm/year in some southeastern areas to over 1,200 mm/year in the west and mountainous areas.
Is this calculated volume actually usable in full?
This calculation gives the theoretically collectible volume over a year, distributed unevenly according to actual rainfall. The storage tank's capacity practically limits the amount actually recovered: a heavy rain event that exceeds the full tank's capacity is lost through overflow, which makes sizing the tank just as important as calculating the theoretical annual volume.
Is this harvested water drinkable?
No, rainwater harvested from a roof is not drinkable without specific treatment and is generally not intended for consumption. It is, however, suitable for many outdoor uses and certain non-food indoor uses (watering, washing, toilet flushing), subject to the local regulations that apply to those uses.