Boneyard Tools

Pounds vs kilograms: the mass units explained

Where the pound and the kilogram come from, why 1 lb equals exactly 0.45359237 kg, and how to convert body weight between them.

Open the Pounds to Kilograms →

Two systems for measuring mass

The pound belongs to the imperial and US customary systems and is still the everyday weight unit in the United States. The kilogram is the base unit of mass in the International System of Units, the metric standard used by most of the world for groceries, medicine and science. Because the two systems grew up separately, the same object can be labeled 150 lb in one country and 68 kg in another. A converter simply restates one measurement in the other unit without changing the actual mass.

Why the factor is exactly 0.45359237

The pound was not always tied so neatly to the metric system. In 1959 the United States and Commonwealth countries signed the international yard and pound agreement, which fixed one avoirdupois pound at exactly 0.45359237 kilograms. That is a defined value, not a measured approximation, so it never changes and carries no uncertainty. This tool uses that exact factor, which is why a round 100 lb converts to a clean 45.359237 kg.

Converting body weight in your head

For a fast mental estimate, divide the pounds by two and then trim off about a tenth. A 160 lb person is roughly 80 kg minus 8, which lands near 72 kg, close to the precise 72.57 kg. Going the other way, double the kilograms and add roughly ten percent to approximate pounds. These shortcuts are handy for a quick sanity check, but for forms, dosing or shipping labels use the exact converted value rather than the rounded estimate.

Pounds, stone and other mass units

In the United Kingdom body weight is often quoted in stone, where one stone equals 14 pounds, so 11 stone is 154 lb or about 69.85 kg. The ounce is a smaller imperial unit at one sixteenth of a pound, while the metric side offers grams and tonnes for very small and very large masses. All of these describe mass rather than force, so a conversion holds true whether you are on Earth, on the Moon or in orbit.

Frequently asked questions

Is a pound a measure of weight or mass?

In everyday use the pound labels mass, the amount of matter in an object, which is what converts directly to kilograms. Physicists sometimes use pound-force for weight, but the avoirdupois pound on a bathroom scale maps to kilograms with the 0.45359237 factor.

How do I convert stone to kilograms?

First turn stone into pounds by multiplying by 14, then multiply the pounds by 0.45359237. For example, 12 stone is 168 lb, which is about 76.203518 kg.