Rainwater from your roof

On average, Germany gets 750 mm of rain a year. From 100 m² of roof, 56,250 to 67,500 litres of it reach the gutter.

Roof area in m² times rain in mm gives litres. Usually 75 to 90% of it arrives; the gutter, splash and evaporation take the rest. We work with a cautious 75%. Outside Germany, use your own annual rainfall in mm.

The footprint under the roof, from eave to eave, not the sloping surface. If only one side drains into the barrel, only that side counts.

Berlin 540 mm a year (station Berlin-Tegel), Munich 940 mm (station München-Stadt), averaged over 1991–2020.

That depends on how often you draw water. As a yardstick we use a rainy day with 10 mm: 20 m² of roof then gives 150 litres. Germany has 19 such days a year on average.