Stone to pounds (st to lb)

One stone is exactly 14 pounds. That ratio belongs to the customary system itself and is much older than any link to metric: the 1959 international yard and pound agreement later fixed the yard and the pound against the meter and the kilogram, but it left the ratios between customary units (feet to the yard, ounces to the pound, cups to the gallon) exactly as they were.

Worked through: 12 st x 14 = 168 lb.

Going the other way, one pound is 0.0714285714 stone.

To convert without a calculator, multiply by 14. Rounding that to 14 is within 0% for most purposes.

Stone is used for body weight in Britain and Ireland and essentially nowhere else. The US does not use it, despite both being imperial. Fourteen pounds to the stone, so 12 stone is 168 lb. An American reading a British weight without converting is out by an order of magnitude.

Precision is worth matching to the input. A measurement given as 8 st carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.