Every modern legal system is made up of two basic kinds of norms: rules and principles. These are applied by means of two different procedures: subsumption and balancing. While rules apply by means of subsumption, balancing is the means of applying principles. Balancing has therefore become an essential methodological criterion for adjudication, especially of constitutional rights. However, balancing is at the heart of many theoretical and practical discussions. One of the most important questions is whether balancing is a rational procedure for applying norms. The aim of this paper is to consider whether this is the case. To achieve this aim, this paper reflects on why the rationality of balancing is in doubt, and to what extent balancing can be rational, and how this can be possible. The weight formula proposed by Robert Alexy is analysed as a model which, in spite of its limits, tries to solve the philosophical and constitutional problems about the rationality of balancing to the greatest extent possible.