It is commonly claimed that the use of numbers and indicators in society is on the rise. Yet, it is still unclear why numbers are used in some social fields but not in others. This paper investigates the specific use of indicators in the policy field of federal spatial planning in Germany. While the use of indicators is very common in this area, the guiding idea of equivalent living conditions has not been operationalized with an agreed on set of indicators nor do particular policy instruments targeting 'unequivalent' living conditions exist. Why is this the case? Drawing on literature about the use of scientific knowledge in politics as well as on social studies of quantification processes, we argue that regular monitoring of regional disparities has increased the political salience of the idea of equivalent living conditions. At the same time, a more systematic formalization of this idea has been hampered, firstly, by the entangled competencies of German federalism and secondly, by the ambiguous idea of equivalent living conditions which has faced competition by the more narrowly defined concept of regional economic development and the related GDP indicator.