The urban heat island, defined as the air temperature difference between the urban canyon and the nearby rural area, is investigated. Since not all cities around the world are equipped with an extensive measurement network, a need exists for a relatively straightforward equation for the urban heat island effect. Here, we derive a simple, diagnostic equation for the urban heat island using dimensional analysis. This equation provides a first order estimation of the daily maximum urban heat island based on routine meteorological observations and straightforward urban morphological properties. The equation is tested for 14 cities across Western-Europe and appears to be robust. The comprehensiveness of this analytical equation allows for applications beyond atmospheric science.