The B6421 is a semi-rural road in the West Riding. It was originally unclassified but had gained its present number by 1932.
The road starts on the A655 to the west of Castleford town centre and heads south along the residential Aketon Road. After passing a leisure centre and going under a dismantled railway line we cross the A639 at traffic lights. The road then continues through the suburbs, bearing left at a roundabout serving a modern housing estate and quickly reaching a roundabout on the A6539. There's then a multiplex ahead along that road which takes us under the M62 to the next roundabout, where the B6421 regains its number by bearing left.
We continue across the fields and quickly enter North Featherstone, crossing the B6134 at traffic lights in the village centre. There's then only a brief glimpse of open country before we reach Featherstone proper. After a sharp right-hand bend the road TOTSOs left at traffic lights and goes over the Wakefield to Pontefract railway line at a level crossing by Featherstone station. A short distance further on, after running through a shopping area, we reach traffic lights on the A645.
There's then a multiplex to the left. At traffic lights the B6421 comes in from the right along a road one-way towards us; outbound traffic must continue slightly further along the A645 before turning right along a two-way road. We soon meet the spur and follow a traffic-calmed route past Purston Park to enter open country, meeting the B6428 immediately. Our road then winds off to the left, descending across the fields to enter High Ackworth, heading east to end on the A628 on the village green.