Rainwater infiltration - how does the calculation work?

Anyone planning a rainwater infiltration must always dimension them correctly. For that you have to know what amount of rainwater is generated at all. How to calculate this correctly, you will learn in detail in our post.

Criteria for the calculation

If you want to calculate the accumulated amount of rainwater, you have to keep an eye on three things:

  • the amount of rain that falls
  • the size of the area to be drained (e.g.
  • Heavy rain events


The amount of rain within Germany can be quite different from area to area. An impression of this can be found in the table below (data from 2016).

stateaverage annual precipitation in l / m²
Baden-Wurttemberg965 l / m² annually
Rheinland-Pfalz800 l / m² annually
Hesse700 l / m² annually
Berlin505 l / m² annually
Saxony-Anhalt475 l / m² annually
Germany - average736 l / m² annually

In order to determine the amount of rain correctly, you therefore always need your location. The annual rainfall (the term is technically correct) can best be determined from the data of the KOSTRA-DWD 2000 (Coordinated Heavy precipitation regionalization evaluations of the German Weather Service). Also in the professional field, these values ​​are used when it comes to calculating the amount of rainwater.

Size of the area to be drained

For example, if you want to dewater the rainwater that hits the roof, you need to know the size of the roof surface. For flat roofs this is still relatively easy, as you only need length and width for the calculation. The pitched roof is a bit more difficult:

You have to multiply the floor area of ​​the house with a certain factor. This factor X, as it is called in roofers, is dependent on the roof pitch and can be looked up in corresponding tables.

Calculation example: Base area of ​​the house = 12 x 6 m
Roof overhang = 0.40 m
Roof pitch = 40°, the associated factor X is 1.3054

Floor area including roof overhang: (12+ (0,4 * 2) x (6 + (0,4 * 2)) = 87,04 m²
Multiplied by the factor X, this gives a roof area of ​​113.62 m²

In addition, the roofing or a corresponding surcharge also plays a role. For this purpose, the roof surface is then multiplied by the so-called discharge coefficient (C), which you can also refer to corresponding tables.


The formula is now Rooftop x Rainfall x Runoff Factor x 1 / 10,000 (if you use the rainfall data above).

Tips & Tricks

Heavy rain events are taken into account in the professional domain in such a way that the strongest event, which occurs every 5 years, is taken as the maximum value.

Video Board: Rational Method Explanation and Example