6.35

The stone was originally used for weighing agricultural commodities. Historically the number of pounds in a stone varied by commodity, and was not the same in all times and places even for one commodity. Potatoes, for example, were traditionally sold in stone and half-stone (14-pound and 7-pound) quantities.
This is defined in British legislation as being a weight or mass equal to 14 avoirdupois pounds (about 6.35 kilograms).