The route begins at the junction with the A474 at the north of Gwaun-Cae-Gurwen, and travels through Lower Brynamman and Brynamman. The route then crosses over the Black Mountain range of the Brecon Beacons and emerges near Felindre near Llangadog. It then crosses through Llangadog and continues northeast until the junction with the A40 at Llandovery. It reaches a height of 493 m (1,617 ft) above sea level.
