Because hypoxia can have catastrophic effects on estuarine ecosystem health, a critical coastal resource management need is the ability to quantify and compare the relative severity of hypoxic events in terms of their potential for ecological impact. This study makes use of continuous, high-frequency water quality monitoring data available through the National Estuarine Research Reserve's System-Wide Monitoring Program to explore a quantitative index approach that captures the transient nature of hypoxic events and allows for their comparison across space and time. The conceptual model explores various time and concentration thresholds for defining “events,” which then allows for ranking their severity in terms of duration and concentration. In our example, we have borrowed from the familiar hurricane categorization index to create an analogous hypoxic event severity index (1–5), with higher values indicating more ecologically damaging events. We demonstrate that the model provides a convenient way to quantify and compare the frequency and severity of hypoxia over 4 years at one site and between two widely separated locations over 3 years.