The B651 is a cross-country B-road in central Hertfordshire.
It starts at a triangular junction on the A1081 (former A6) to the north of St Albans city centre. It heads northeast along Sandridge Road to leave town, crossing the Midland Main Line en route. There is a short semi-rural section before the village of Sandridge but open country is not actually reached until after that village.
The road crosses Nomansland Common to reach Wheathampstead and descends to a mini-roundabout at the southern end of the High Street. This was once the start of a multiplex with the B653 but a new bypass has been built taking that road out of the village centre, which is a 20 zone. After crossing the River Lea the 20 zone ends and the road meets the current route of the B653 at a roundabout on the edge of the village.
The B651 continues north. It passes between two golf courses then enters a wood where it turns right at a badly signed TOTSO. It then descends the narrow Ballslough Hill into Kimpton where it TOTSOs left at a T-junction into the High Street then takes the next right turn, with the road ahead being the B652. The road goes over another hill into Whitwell where once again it reaches a T-junction on the High Street - it TOTSOs right this time. It heads east for a few hundred yards before having to TOTSO left once more to regain its northerly route.
The next village is St Paul's Walden, childhood home of Elizabeth Bowes-Lyon, wife of George VI. After going round Hitch Wood the B651 ends at a T-junction on the B656 (former A600) a short distance to the north of Rush Green.