Pound (lb)

The pound is a unit of mass equal to exactly 0.45359237 kilograms. Its symbol is lb, from the Latin libra, and the plural is sometimes written lbs. It is the principal unit of mass in the United States and remains widely used in the United Kingdom for body weight and food.

That exact value dates from the International Yard and Pound Agreement of 1959, in which the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa agreed a single definition for both units. Before it, the American and British pounds differed slightly, which was tolerable in commerce and unacceptable in engineering as aircraft and munitions were built to shared drawings. Defining the pound in terms of the kilogram, rather than the other way round, quietly made the imperial system a derivative of the metric one.

The abbreviation lb is a reminder of the Roman libra pondo, a pound by weight, from which English took the word pound and the symbol from different halves of the phrase. The same libra gave the pound sterling its name and its old symbol, which is why a currency and a weight share a word.

Everyday American life is dense with the unit. Body weight, groceries sold loose, meat, flour and sugar, freight, and gym equipment are all in pounds. Weight-class sports set limits in pounds, and infant birth weights are given in pounds and ounces. In the United Kingdom the pound survives socially rather than commercially: shops sell in kilograms while people still describe their own weight in stones and pounds.

A second pound causes endless trouble in engineering. The pound-force is the weight of one pound of mass in standard gravity, about 4.448 newtons, and it is written with the same abbreviation in most American practice. Since force equals mass times acceleration, using the two interchangeably requires a conversion constant, and generations of textbooks have handled that either by introducing the slug — the mass that one pound-force accelerates at one foot per second squared, about 14.59 kilograms — or by carrying a factor of 32.174 through every calculation. Neither is elegant, and the confusion is the strongest practical argument the metric system has ever had in its favour.

One pound equals 0.45359237 kilograms exactly, 16 ounces, or 7000 grains.

Pound conversion table