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
The word pound comes from the Latin word pendere, meaning "to weigh". The Latin word libra means "scales, balances" and it also describes a Roman unit similar to the pound-force. This is the origin of the abbreviation "lb" for the pound.
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 £79 or about $158 US today). In 1528, the standard was changed to the Troy pound (worth about £84 or $168 today).
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 , which would make the kilogram approximately equal to . (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 was a better approximation. With the Weights and measures Act 1889 the United Kingdom legally defined the avoirdupois pound as the rounded value of .
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 per kilogram. In 1894, the relationship was specified to be 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.
In the United Kingdom, the use of the international pound was implemented in the Weights and Measures Act 1963.
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
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
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:
|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|
|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 livre esterlin was equivalent to about and was used between the late ninth and the mid-fourteenth centuries.
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.. 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.
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.
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..."