SR 638 has its southern terminus at an intersection with U.S. Route 68 (US 68) just over six miles (9.7 km) north of the city of Bellefontaine.

The northern terminus of the route is at SR 273 at the extreme southeastern corner of the village of Belle Center.

[2] The highway begins at a sharp T-intersection with US 68 in northeastern McArthur Township under 765,000-volt power lines.

Starting out traveling in a north-northeasterly direction, SR 638 passes through a backdrop of farmland for its entire duration, with the occasional patch of trees and the occasional home appearing alongside the roadway.

The route meets SR 274, passes by Fulton Cemetery, then bends to the north-northwest.