One stone is exactly 14 pounds. Both units are avoirdupois, so the conversion is exact and simple, and the pair exists almost entirely to express British body weight.
About the units
The stone as a trading weight varied wildly by commodity before standardisation — the wool stone, the cheese stone and the glass stone were all different, ranging from about 5 to 40 pounds. The Weights and Measures Act of 1835 fixed the general stone at 14 pounds, and it is that value the unit still carries. The pound is the avoirdupois pound, exactly 0.45359237 kg. Fourteen is an unusual base and it has no deeper logic; it is simply the value that survived standardisation from among many competing local definitions. The stone was withdrawn from legal trade use in the United Kingdom in 1985, so nothing can be sold by it, but no law governs how people describe their own weight.
The exact factor
The factor is exactly 14. Weight in this system is written as whole stones plus a remainder in pounds, never as decimal stone, so 170 pounds is 12 stone 2 pounds: divide by 14 for whole stones, keep the remainder as pounds. The most common mistake is reading a decimal as pounds — 12.2 stone is 12 stone and 2.8 pounds, not 12 stone 2 pounds, and the two differ by nearly a pound. In kilograms, one stone is 6.35029318 kg and one pound is 0.45359237 kg.
Where you meet this conversion
British and Irish body weight, in conversation, on bathroom scales, in health guidance and in sport reporting. British boxing and horse racing both use stones and pounds, the latter for jockey weights including equipment. Weight-loss programmes market progress in stones because the number moves slowly enough to feel meaningful. Anyone reading British sources internationally meets the unit constantly and needs the fourteen to make sense of it, since no other everyday system divides by that number.