European integration is a multilayer process consisting of significant differences in efforts and capabilities of the Member State's individual EU participation. Hence, general statements about the national level of European economic integration are very vague. In order to fill this gap, this article presents a composite indicator measuring the extent of economic integration within the European Union – the EU Index. Existing composite indicators concerned with economic integration (globalization indices) were not designed to capture the specific European dimensions. The EU Index offers a unique basis, as now the national differences can be illustrated by one statistical measure. Large heterogeneities are found between the Member States with respect to overall European economic integration and to various sub‐indices. By using cluster analysis, it is also shown that the prevailing economic heterogeneities in the EU are combined with a strong and even growing clustering of its members, thereby challenging present and future steps of European integration.