The pound or pound-mass (abbreviation: lb, lbm, or sometimes in the United States: #) 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 kilogram.
The word pound comes from the Latin word pendere, meaning "to weigh". The abbreviation lb comes from the Latin word libra, meaning "scales, balances", which also described a Roman unit similar to the pound.
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Historically, in different parts of the world, at different points in time, and for different applications, the pound (or its translation) has referred to broadly similar but not identical standards of mass or weight.
A number of different definitions of the pound have been used in Britain. Amongst these are the avoirdupois pound and the obsolete Troy, tower, merchant's and London pounds.[1]
There is a historical link between the pound as a unit of mass and the pound as a unit of currency. Originally the pound sterling was equivalent to the value of a Tower pound of silver (worth about £56 or about $92 US[2] today). In 1528, the standard was changed to the Troy pound (worth about £60 or $99 today).
| Unit | Pounds | Ounces | Grains | Metric | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| avdp. | troy | tower | merc. | lond. | metric | avdp. | troy | tower | g | kg | ||
| Avoirdupois | 1 | 175⁄144 | 35⁄27 | 28⁄27 | 35⁄36 | ~10⁄11 | 16 | 147⁄12 | 155⁄9 | 7000 | ~454 | ~9⁄20 |
| Troy | 144⁄175 | 1 | 16⁄15 | 64⁄75 | 5⁄6 | ~3⁄4 | 1329⁄175 | 12 | 124⁄5 | 5760 | ~373 | ~3⁄8 |
| Tower | 27⁄35 | 15⁄16 | 1 | 4⁄5 | 3⁄4 | ~7⁄10 | 1212⁄35 | 111⁄4 | 12 | 5400 | ~350 | ~7⁄20 |
| Merchant | 27⁄28 | 75⁄64 | 5⁄4 | 1 | 15⁄16 | ~7⁄8 | 153⁄7 | 141⁄16 | 15 | 6750 | ~437 | ~7⁄16 |
| London | 36⁄35 | 6⁄5 | 4⁄3 | 16⁄15 | 1 | ~14⁄15 | 1616⁄35 | 15 | 16 | 7200 | ~467 | ~7⁄15 |
| Metric | ~11⁄10 | ~4⁄3 | ~10⁄7 | ~8⁄7 | ~15⁄14 | 1 | ~173⁄5 | ~16 | ~171⁄7 | ~7716 | 500 | 1⁄2 |
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. 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.[3]
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 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 0.453592427 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.[4]
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.[4]
According to a 1959 NIST publication, the international pound differed from the United States 1894 pound by approximately one part in 10 million.[5]. The difference is so insignificant that it can be ignored for almost all practical purposes.[6]
In 1958 the United States and countries of the Commonwealth of Nations agreed upon common definitions for the pound and the yard. The international avoirdupois pound was defined as exactly 453.59237 grams.
In the United Kingdom, the use of the international pound was implemented in the Weights and Measures Act 1963.[7]
“ The yard or the metre shall be the unit of measurement of length and the pound or the kilogram shall be the unit of measurement of mass by reference to which any measurement involving a measurement of length or mass shall be made in the United Kingdom; and- (a) the yard shall be 0·9144 metre exactly; (b) the pound shall be 0·453 592 37 kilogram exactly.”
- Weights and Measures Act, 1963, Section 1(1)
An avoirdupois pound is equal to 16 avoirdupois ounces and to exactly 7,000 grains. The conversion factor between the kilogram and the international pound was therefore chosen to be divisible by 7, and an (international) grain is thus equal to exactly 64.79891 milligrams.
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 373.241 721 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. In the United Kingdom, the use of the troy pound was abolished on 6 January 1879. The troy ounce is still used for measurements of precious metals such as gold, silver, and platinum, and sometimes gems such as opals.
Most measurements of the mass of precious metals using pounds refer to troy pounds, even though it is not always explicitly stated that this is the case. Some notable exceptions are:
The tower pound was based on the wheat grain unlike all the other English measures, where the grain was based on the barley grain.
| 1 tower pound | = | 7,200 tower grains | = | 5,400 troy grains |
| 1 tower ounce | = | 600 tower grains | = | 450 troy grains |
| 1 tower pennyweight | = | 30 tower grains | = | 22½ troy grains |
The merchants' pound (mercantile pound, libra mercantoria or commercial pound) was equal to 9,600 wheat grains (15 tower ounces or 6,750 grains). It was used in England until the 14th century for most goods (other than money, spices and electuaries).[8]
A London pound was equal to 7,200 troy grains (16 tower ounces or, equivalently, 15 troy ounces).
| 1 London pound | = | 1⅓ tower pounds | = | 7,200 troy grains |
| 1 London ounce | = | 1 tower ounce | = | 450 troy grains |
| 1 London pennyweight | = | 1 tower pennyweight | = | 22½ troy grains |
The wool pound was equal to 6,992 grains. It was a unit of mass used to measure the quantity of wool.[9]
The libra (Latin for "scales / balance") is an ancient Roman unit of mass that was equivalent to approximately 327 grams. It was divided into 12 uncia, or ounces. The libra is the origin of the abbreviation for pound, lb.
Since the Middle Ages various pounds have been used in France. The word pound translates to livre in French, a word which continues to be used today to refer to a metric pound.
The livre esterlin was equivalent to about 367.1 grams (5,665 gr) and was used between the late ninth and the mid-fourteenth centuries.[10]
The livre poids de marc or livre de Paris was equivalent to about 489.5 grams (7,555 gr) and was used between the 1350s and the late 18th century.[10]. It was introduced by the government of John II.
The livre métrique was set equal to the kilogram by the decree of 13 Brumaire an IX between 1800 and 1812. This was a form of official metric pound.[10]
The livre usuelle was set equal to 500 grams, by the decree of 28 March 1812. It was abolished as a unit of mass effective 1 January 1840 by a decree of 4 July 1837.[10]
The Russian pound (Фунт, funt) is an obsolete Russian unit of measurement of mass. It is equal to 409.5124 g.
A Jersey pound is an obsolete unit of mass used on the island of Jersey from the 14th century to the 19th century. It was equivalent to about 7,561 grains. It may have been derived from the French livre poids de marc.[11]
The trone pound is one of a number of obsolete Scottish units of measurement. It was equivalent to between 21 to 28 avoirdupois ounces.
In many countries upon the introduction of a metric system, the pound (or its translation) became an informal term for half of a kilogram or 500 grams, often following an official redefinition of an existing unit during the 19th century. The Dutch pond is an exception. It was officially redefined as 1 kilogram, with an ounce of 100 grams, but people seldom use it this way. In daily life pond is exclusively used for amounts of 500 grams, and to a lesser extent, ons for 100 grams.
In German the term is Pfund, in French livre, in Dutch pond, in Spanish and Portuguese libra, in Italian libbra, and in Danish pund.
Hundreds of older pounds were replaced in this way. Examples of the older pounds are one of around 459 to 460 grams in Spain, Portugal, and Latin America; one of 498.1 grams in Norway; and several different ones in what is now Germany.
Although the use of the pound as an informal term persists in these countries to a varying degree, scales and measuring devices are denominated only in grams and kilograms. A pound of product must be determined by weighing the product in grams. The use of the term pound is usually forbidden for official use in trade.
In the United States of America the United States Department of Commerce, the Technology Administration, and the National Institute of Standards and Technology (NIST) have defined the use of mass and weight in the exchange of goods under the Uniform Laws and Regulations in the areas of legal metrology and engine fuel quality in NIST Handbook 130.
NIST Handbook 130 states:
- V. "Mass" and "Weight." [NOTE 1, See page 6]
- The mass of an object is a measure of the object’s inertial property, or the amount of matter it contains. The weight of an object is a measure of the force exerted on the object by gravity, or the force needed to support it. The pull of gravity on the earth gives an object a downward acceleration of about 9.8 m/s2. In trade and commerce and everyday use, the term "weight" is often used as a synonym for "mass." The "net mass" or "net weight" declared on a label indicates that the package contains a specific amount of commodity exclusive of wrapping materials. The use of the term "mass" is predominant throughout the world, and is becoming increasingly common in the United States. (Added 1993)
- W. Use of the Terms "Mass" and "Weight." [NOTE 1, See page 6]
- When used in this handbook, the term "weight" means "mass." The term "weight" appears when inch-pound units are cited, or when both inch-pound and SI units are included in a requirement. The terms "mass" or "masses" are used when only SI units are cited in a requirement. The following note appears where the term "weight" is first used in a law or regulation.
- NOTE 1: When used in this law (or regulation), the term "weight" means "mass." (See paragraph V. and W. in Section I., Introduction, of NIST Handbook 130 for an explanation of these terms.) (Added 1993) 6"
U.S. federal law, which supersedes this handbook, also defines weight, particularly Net Weight, in terms of the avoirdupois pound or mass pound. From 21CFR101 Part 101.105 – Declaration of net quantity of contents when exempt:
- (a) The principal display panel of a food in package form shall bear a declaration of the net quantity of contents. This shall be expressed in the terms of weight, measure, numerical count, or a combination of numerical count and weight or measure. The statement shall be in terms of fluid measure if the food is liquid, or in terms of weight if the food is solid, semisolid, or viscous, or a mixture of solid and liquid; except that such statement may be in terms of dry measure if the food is a fresh fruit, fresh vegetable, or other dry commodity that is customarily sold by dry measure. If there is a firmly established general consumer usage and trade custom of declaring the contents of a liquid by weight, or a solid, semisolid, or viscous product by fluid measure, it may be used. Whenever the Commissioner determines that an existing practice of declaring net quantity of contents by weight, measure, numerical count, or a combination in the case of a specific packaged food does not facilitate value comparisons by consumers and offers opportunity for consumer confusion, he will by regulation designate the appropriate term or terms to be used for such commodity.
- (b)(1) Statements of weight shall be in terms of avoirdupois pound and ounce.
See also 21CFR201 Part 201.51 – "Declaration of net quantity of contents" for general labeling and prescription labeling requirements.
From paragraph "a" above, although the avoirdupois pound is a measure of mass, in commerce it is used with the term "Net Weight", because "there is a firmly established general consumer usage and trade custom of declaring the contents of a liquid by weight, or a solid..."