Existing formula, like the well-known Penman-Monteith equation, for calculating evapotranspiration possess great differences concerning their results. Causes for that are the different climatic conditions and vegetation specifics during their development. Every formula of evapotranspiration only delivers 100 % correct results, if it is used under the same climatic condition it was developed in. Therefore a new method for calculating evapotranspiration via specific, but comparatively easy established, formula is presented. After a theoretical introduction for illustrating some fundamental aspects for this work, the test setup and approach for creating these formula are described in detail. The test set up considers typical climatic conditions and simulates, for example, the average temperature of Vienna, a summer day and a dessert day. While using a polynomial correlation, a formula for calculating evapotranspiration at any temperature is possible. For determining the evapotranspiration rate (in mm/m² per 24 hours day) due to the presented formula, only temperature, sort of vegetation and technical soil has to be defined. As a result, the relevance of this work is the presentation of a method to determine the evapotranspiration of any technical substrates (used for e.g. greenroofs, façade greenery, indoor greenery, raingardens,…) with or without any kind of vegetation, while using the created formula.