The B478 is a short B-road through the village of Sonning in Berkshire.
It starts at a roundabout in Charvil, at the western end of the A4 Twyford bypass (the A3032 through the town also meets the roundabout). The road heads west into Sonning, passing Sonning Farm (part of the University of Reading). After crossing a mini-roundabout (where it meets the B4446) the B478 passes the church and then crosses the navigable channel of the River Thames. The bridge is narrow and therefore controlled by traffic lights.
After passing the mill the original river channel is crossed (by a wider bridge) and the road bypasses Sonning Eye (where a roundabout gives access to the original road through the hamlet). The B478 skirts a flooded gravel pit before ending on a roundabout on the A4155 (original A32).