The pound is defined in kilograms
The international avoirdupois pound has been exactly 0.45359237 kilograms since the 1959 agreement between the English-speaking nations. As with the inch, this is a definition rather than a measurement, so the conversion carries no uncertainty. One kilogram works out at 2.2046226… pounds.
Why stone appears here
In the UK and Ireland body weight is normally given in stone and pounds — fourteen pounds to the stone — and almost nowhere else. Someone weighing 70 kg is 154.3 lb, which a British reader would immediately restate as 11 stone 0.3 pounds. The stone row exists because that translation is the second half of the question for a large share of people asking it.
Mass, not weight
Strictly, kilograms and pounds as used here are units of mass, and both would be unchanged on the Moon. The pound-force is a separate unit of weight that happens to share the name, which is why engineering texts write “lbm” and “lbf” to keep them apart. For scales, shipping and body weight the distinction never arises, because everything is being weighed in the same gravitational field.
Rough conversion in your head
Doubling and adding ten percent gets you very close: 70 kg → 140 + 14 = 154 lb, against a true 154.3. Halving and knocking off ten percent works in reverse: 154 lb → 77 − 7.7 = 69.3 kg, against a true 69.9. Both are within a percent, which is well inside the accuracy of most bathroom scales.