The clearance height under a bridge is the maximum height allowed for passage beneath its span. If the bridge passes over water of variable level, the normally occurring high water mark is used. High water is defined as the high tide mark in areas affected by tides, and the normal seasonal high water mark in lakes, rivers, streams, etc.

The water reference is the normally occurring high level, so extra clearance visible at low tide does not increase this value. An exceptional flood is separate from the normal seasonal high-water reference.

This measurement concerns passage below the structure. The vehicle clearance on a bridge deck concerns traffic crossing above it.