Pounds to kilograms
One pound equals 0.45359237 kilograms, a figure fixed by international agreement in 1959 rather than measured. Multiply pounds by 0.45359237 for kilograms. A useful mental check: halve the pounds and take off ten per cent, which puts 150 lb at about 67.5 kg against a true 68.04 kg.
How to convert pounds to kilograms
Gym plates are the place this bites. American plates come in 45, 35, 25, 10, 5 and 2.5 pounds; European bars are loaded in 20, 15, 10, 5 and 2.5 kilograms. A 45 lb plate is 20.41 kg, not 20, so a bar loaded to "225 lb" is 102.06 kg rather than the round 100 kg somebody will quote you.
Questions
The 1959 international yard and pound agreement, signed by six countries, defined the avoirdupois pound as exactly that many kilograms. It is a definition, so it has no error bar.
One stone is 14 pounds, so 6.35 kg. British body weight is often quoted in stones and pounds together: convert the whole thing to pounds first, then use this.
Yes for everyday use. Both use the international avoirdupois pound. The troy pound, still used for precious metals, is a different unit at 373.24 grams.
A 50 lb bag limit is 22.68 kg, which airlines usually round down to 23 kg. Two systems, two roundings, and occasionally an argument at the desk.