Success in software development depends on the availability of complete, consistent, and unambiguous functional software requirements. Inconsistencies in software requirements can propagate problems throughout the development cycle. We introduce the concept of a quantitative measure for detecting inconsistencies, namely, Potential Structural Inconsistency (PSI). This measure is derived from a structural model for a given set of requirements. We show how this measure can be determined using a case study with known inconsistencies.