Running for about 2 miles (3.2 km) in a general east–west direction, connecting Naval Surface Warfare Center Crane Division to U.S. Route 231 (US 231).

The road passes through woodland with some farmland on its way towards the unincorporated census-designated place of Burns City.

The highway leaves Burns City and quickly meets an entrance to the Naval Surface Warfare Center Crane Division.

[2][3] The traffic count on SR 645 is 1,624 vehicles travel the highway on average each day in 2016.

This reroute removed a state road designation from the modern SR 645 routing.