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The pound or pound-mass (abbreviation: lb, lbm) is a unit of mass used in the imperial, United States customary and other systems of measurement. A number of different definitions have been used, the most common today being the international avoirdupois pound of exactly 0.45359237 kilograms.

The unit is descended from the Roman libra (hence the abbreviation lb); the name pound is a Germanic adaptation of the Latin phrase libra pondo 'a pound weight'.[1]

The pound is the force of 0.453 592 37 kg subject to a standard gravity of 9.80665 m s−2, or approximately 4.448 N

The avoirdupois pound was invented by London merchants in 1303. Originally it was based on independent standards. During the reign of Henry VIII of England, the avoirdupois pound was redefined as 7,000 troy grains.[citation needed] Since then, the grain has often been considered as a part of the avoirdupois system. By 1758, two standard weights for the avoirdupois pound existed, and when measured in troy grains they were found to be of 7,002 grains and 6,999 grains.[4]

In the United Kingdom, the avoirdupois pound was defined as a unit of mass by the Weights and Measures Act of 1878, but having a different value (in relation to the kilogram) than it does now, of approximately 0.453592338 kg, which would make the kilogram approximately equal to 2.20462278 pounds. (This was a measured quantity, with the independently maintained artifact still serving as the official standard[citation needed] for this pound.) This old value is sometimes called the imperial pound, and this definition and terminology are obsolete unless referring to the slightly-different 1878 definition. In 1883 it was determined that 0.4535924277 kg was a better approximation. With the Weights and measures Act 1889 the United Kingdom legally defined the avoirdupois pound as the rounded value of 0.45359243 kg.[5]

In the United States, the (avoirdupois) pound as a unit of mass has been officially defined in terms of the kilogram since the Mendenhall Order of 1893. In 1893, the relationship was specified to be 2.20462 pounds per kilogram. In 1894, the relationship was specified to be 2.20462234 pounds per kilogram. This change followed a determination of the British pound.[5]

According to a 1959 NIST publication, the international pound differed from the United States 1894 pound by approximately one part in 10 million.[6] The difference is so insignificant that it can be ignored for almost all practical purposes.[7] [edit] International pound

The United States and countries of the Commonwealth of Nations agreed upon common definitions for the pound and the yard. Since 1 July 1959, the international avoirdupois pound has been defined as exactly 0.45359237 kilogram.[8]

The troy pound takes its name from the French market town of Troyes in France where English merchants traded at least as early as the time of Charlemagne (early ninth century). The system of Troy weights was used in England by apothecaries and jewellers.

A troy pound is equal to 12 troy ounces and to 5,760 grains. Today, the grain is common to the avoirdupois and troy systems of units of mass making an international troy pound equal to 373.2417216 grams.

The troy pound is no longer in general use. In Canada, Australia, the United Kingdom, and other places the troy pound is no longer a legal unit for trade.[citation needed] In the United Kingdom, the use of the troy pound was abolished on 6 January 1879.