The objective of Air Traffic Flow Management is to maintain safe andefficient use of airspace and airports by regulating the flow oftraffic. In this paper,we introduce a single-valued metric for post-operatively rating theperformance ofachieved traffic flow against targeted traffic flow. We providevariations on themetric, one of which factors out stochastic conditions upon which a planis formu-lated, and show how these improve on current traffic control analysistechniques.The core of the metric is intuitive and simple, yet leads to aninteresting optimiza-tion problem that can be efficiently solved via dynamic programming.Numericalresults of the metric are given as well as a sample of the type ofanalysis that shouldfollow a low rating by the metric. Although this metric was originallydevelopedto rate the performance of Ground Delay Programs, it is equallyapplicable to anysetting in which the flow of discrete objects such as vehicles iscontrolled and laterevaluated.