Kilograms to pounds

The formula

pounds = kilograms × 2.2046226218
kilograms = pounds × 0.45359237

The factor 0.45359237 is exact by definition. Since 2019, the kilogram itself has been defined by fixing the Planck constant rather than by a physical artefact - the platinum-iridium cylinder in Sèvres that served as the standard for 130 years was retired because it was measurably losing mass.

Quick reference

kg	lb	kg	lb
1	2.205	60	132.28
5	11.023	70	154.32
10	22.046	80	176.37
20	44.092	90	198.42
30	66.139	100	220.46
40	88.185	110	242.51
50	110.23	120	264.55

Kilograms to stone and pounds

kg	Stone and pounds
50	7 st 12.2 lb
60	9 st 6.3 lb
65	10 st 3.3 lb
70	11 st 0.3 lb
75	11 st 11.4 lb
80	12 st 8.4 lb
90	14 st 2.4 lb
100	15 st 10.5 lb

To do this yourself: convert to pounds, divide by 14 for the stone, and the remainder is the pounds.

Mass and weight are not the same thing

Kilograms and pounds are used interchangeably in daily life, but they measure different quantities in physics.

Mass is how much matter something contains. It does not change with location.

Weight is the force gravity exerts on that mass. It changes with location - an object weighs about 16.5% less at the lunar surface than on Earth, while its mass is identical.

The kilogram is strictly a unit of mass. The pound is used for both, with pound-force distinguished as a separate unit in engineering. Because everyday weighing happens on a planet whose gravity is effectively constant, the distinction almost never matters outside a physics classroom - but it is why a scale is technically measuring mass while calling it weight.

Mental arithmetic

kg → lb: double, then add 10%. 85 kg → 170 + 17 = 187 lb. (True: 187.39)

lb → kg: halve, then subtract 10%. 180 lb → 90 − 9 = 81 kg. (True: 81.65)

Both approximations run about 0.2% low, which is well within the tolerance of any bathroom scale.

Where each unit is used

The kilogram is the SI base unit and standard almost everywhere. Pounds persist in the United States for body weight, food packaging and freight. The UK occupies a middle ground: shops sell in kilograms by law, while people describe their own weight in stone and their babies’ in pounds and ounces.

For anything medical, use kilograms. Drug dosing is calculated per kilogram of body mass worldwide, and unit confusion in that context is a documented cause of serious harm.