This paper develops an index for estimating the position of the European Union in various policy fields and ideological dimensions. The index is designed for macro-comparative studies, which currently mainly use a dummy variable as a proxy for the impact of the EU on domestic politics, policies and outcomes. The index assesses the position of each European institution (European Council, Council of Ministers, Commission, and European Parliament) and takes the frequently changing decision-making rules between these institutions into account. Based on an actor-centered approach the index links policy preferences of actors with formal and informal decision-making rules. The index thereby appraises the impact of the EU on its member states in greater detail and is therefore suitable for analyses of most urgent research questions concerning the relationship between the EU and its member states. Furthermore, the index allows a calculation of the positional and ideological misfits between the EU and each individual member state on an annual basis. Questions about an ideological bias of the EU or problems of compliance can thus be analyzed with greater precision than before. To illustrate our index we use data on the left-right dimension, which has proven to be highly relevant in macro-quantitative studies. The data is available for quarterly and annual longitudinal analyses from 1966 to 2012.