🌧️ Rainwater Collection Calculator
Enter your roof's footprint area, the rainfall and a runoff coefficient to see how much water you can collect, in litres and US gallons.
Harvestable water
2,250 L
In US gallons
594.4 gal
Rainwater harvested = roof footprint area × rainfall depth × runoff coefficient. One millimetre of rain on one square metre is exactly one litre; one inch on one square foot is about 0.623 gallons. Use the horizontal footprint (not the sloped surface), and a runoff coefficient around 0.8–0.9 for a typical roof to account for splash, evaporation and first-flush losses. 🔒 In your browser.
How the rainwater collection calculator works
The water a roof sheds is its horizontal footprint area × the rainfall depth × a runoff coefficient (for losses). The neat identity is that one millimetre of rain over one square metre is exactly one litre, and one inch over one square foot is about 0.623 US gallons. The tool converts your area and rainfall to consistent units, applies the runoff coefficient, and reports litres and gallons. Metric or imperial inputs both work.
Use the roof's horizontal footprint (the area it covers on the ground), not the sloped surface area — a pitched roof catches rain according to its footprint. A runoff coefficient around 0.8–0.9 is typical for a solid roof, accounting for splash, evaporation, wetting and first-flush diversion; rougher or more absorbent surfaces collect less.
Frequently asked questions
How much rainwater can I collect from my roof?
Multiply the roof's footprint area by the rainfall depth by a runoff coefficient. As a rule of thumb, 1 mm of rain on 1 m² gives 1 litre, so a 100 m² roof in 25 mm of rain yields about 2,250 litres at 0.9 efficiency.
How do you calculate rainwater harvesting?
Harvest = catchment area × rainfall × runoff coefficient. Keep units consistent (the tool handles m²/ft² and mm/inches), and use a coefficient of about 0.8–0.9 for a typical roof to allow for losses.
Do I use the sloped or flat roof area?
The flat (horizontal) footprint — the area the roof covers when viewed from above. Rain falls vertically, so a pitched roof collects according to its footprint, not the larger sloped surface.
What is a runoff coefficient?
A factor (0–1) for how much rain actually reaches your tank after splash, evaporation, surface wetting and first-flush diversion. Smooth metal roofs are high (~0.9); rougher or absorbent surfaces are lower.
How much is one inch of rain on my roof?
About 0.623 US gallons per square foot. So an inch of rain on a 1,000 ft² roof is roughly 623 gallons before losses — multiply by your runoff coefficient for the usable amount.