The U.S. bureaucracy routinely issues major public policy decisions that affect Americans’ lives. Government agency leaders make those decisions based on a subjective understanding of their agency’s available policy discretion. Over time, discretion has become a prominent theoretical construct in the bureaucratic politics and public administration literatures, but it is rarely measured directly. In this article, we create a new measure of agency policy discretion. We draw on research suggesting that discretion is derived from the bureaucracy’s key political principals: the elected executive, legislators, and interest groups. We use data from the American State Administrators Project and trigonometry to calculate the discretion area scores for 8,955 state agencies between 1978 and 2018. We then evaluate the measure through a series of construct validation assessments. The article offers an innovative and generalizable way to operationalize discretion that will advance future scholarship in organizational behavior, public administration, and bureaucratic decision-making.