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.
Usage of the unqualified term pound reflects the historical conflation of mass and weight resulting from the near uniformity of gravity on Earth. This accounts for the modern distinguishing terms pound-mass and pound-force.
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 tower, merchant's and London pounds. The weight of precious metals when given in pounds and/or ounces usually assumes Troy pounds and ounces; these units are not otherwise used today.
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.
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 0.4535924277 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.
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.
According to a 1959 NIST publication, the international pound differed from the United States 1894 pound by approximately one part in 10 million. The difference is so insignificant that it can be ignored for almost all practical purposes.
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.
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 0·45359237 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 373.2417216 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. 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 system was based on the wheat grain (~50 mg), unlike all the other English systems of weight measurement, which were based on the barley grain (~65 mg). It was the system of measurement used by the Royal Mint. It was abolished in 1527.
|1 tower pound (12 oz)||=||7,200 tower grains||=||5,400 troy grains|
|1 tower ounce (20 dwt)||=||600 tower grains||=||450 troy grains|
|1 tower pennyweight (dwt)||=||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).
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 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. The commonly used abbreviation lbs to indicate the plural unit of measurement does not reflect Latin usage, in which lb is both the singular and plural abbreviation.
Since the Middle Ages, various pounds (livre) have been used in France. Since the nineteenth century, a livre has referred to the metric pound, 500g.
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.
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.
Originally derived from the Roman libra, the definition varied throughout Germany in the Middle Ages and onward. While a Pfund might equal 510 grams in Nuremberg, it was only 467 grams in Berlin. In 1854 the German Customs Union decided that a Pfund must equal 500 grams in all of Germany. The Pfund is no longer an official measurement in Germany, but is still quite often used in everyday speech when buying food to describe half a kg.
A Scandinavian measurement which varied in weight between regions. From the 17th century onward it was equal to 425.076 grams in Sweden. It was abandoned in 1889 when Sweden switched to the metric system. In Norway the same name was used for a weight of 498.1 grams, and in Denmark it equalled 471 grams. In the 19th century Denmark followed Germany's lead and redefined the pound as 500 grams.
20 skålpund = 1 lispund
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 (490 grams). It may have been derived from the French livre poids de marc.
The trone pound is one of a number of obsolete Scottish units of measurement. It was equivalent to between 21 to 28 avoirdupois ounces (about 600-800 grams).
In many countries upon the introduction of a metric system, the pound (or its translation) became an informal term for 500 grams (half a kilogram, similar to the metric pint, being half a litre), 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.
Though not from the same linguistic origin, the Chinese jin (also known a "catty") has a modern definition of exactly 500 grams, divided into ten cun. Traditionally about 605 grams, the jin has been in use for more than two thousand years, serving the same purpose as "pound" for the common-use measure of weight.
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..."
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