Over the past decade, an increasingly sophisticated literature has sought to capture the nature, sources, and consequences of a novel empirical phenomenon in world politics: the growing complexity of global governance institutions. However, this literature has paid only limited attention to questions of measurement, which is a prerequisite for a more comprehensive understanding of global governance complexity across space and time. In taking a first step in this direction, the present article makes two contributions. First, we propose new quantitative measures that gauge the extent of complexity in global governance, which we conceptualize as the degree to which global governance institutions overlap. Dyadic, weighted, directed-dyadic, and monadic measures enable a multifaceted understanding of this important development in world politics. Second, we illustrate these measures by applying them to an updated version of the most comprehensive dataset on the design of intergovernmental organizations (IGOs): the Measure of International Authority (MIA). This allows us to identify cross-sectional and temporal patterns in the extent to which important IGOs, which tend to form the core of sprawling regime complexes in many issue areas, overlap. We conclude by outlining notable implications for, and potential applications of, our measures for research on institutional design and evolution, legitimacy, and legitimation, as well as effectiveness and performance. This discussion underscores the utility of the proposed measures, as both dependent and independent variables, that allow researchers to examine the sources and consequences of institutional overlap in global governance and beyond.