EN 13791 applies when assessing the in situ compressive strength of structures and precast concrete components. According to the code itself, it may be adopted when doubt arises about the compressive strength of a concrete. For assessing the structural safety of existing structures, however, the method given in EN 13791 does not seem to be applicable and may lead to an unsafe approach for determining the characteristic concrete strength. This paper presents an alternative method for determining the characteristic concrete strength from cylinders obtained in situ. The method proposed is based on EN 1990 (Eurocode basis of design) and the corresponding Annex D. The method according to EN 13791 is outlined in this article. Moreover, the practical implementation of the method in accordance with clause 5.2 of EN 1990 is explained and an example is given. Finally, both methods are compared with each other. It is demonstrated that EN 13791 does not apply to safety assessments of existing concrete structures and the use of this code may lead to unsafe situations.