The route starts at the Ballagawne Roundabout on the A7, opposite the A36, and is therefore really a continuation of the A36. The route heads south, very quickly curving round to the south west as it follows Church Road past a few houses and through a nature reserve. It then passes the parish church, with it's extensive graveyards, at the junction with the B46, and enters Port Erin soon after. Running along the eastern edge of the town, the route passes modern housing estates to reach the Four Roads Roundabout, where it crosses the A5. Continuing ahead, the route curves round to the south east and passes a few fields as it leaves Port Erin behind and enters Port St Mary, crossing the steam railway at a level crossing along the way. The route then ends at a crossroads with the A31, and maintains priority as it becomes the B18 into the town.
As originally numbered in the 1920's, the northern section of the route, between Ballagawne and Four Roads was given the B23 number. It had been renumbered as the A29 by 1963. At that time, the A29, however, continued into Port St Mary, on what is now the B18, following Station Road and Bay View Road into the town centre, and then High Street and Athol Street on to the harbour. Here it doubled back on Queens Road and then followed what is now the B28 along Fistards Road and Park Road to form a loop, ending on itself. It is not known when this extension occurred, whether before or after the renumbering.