The pound (abbreviations: lb or, sometimes in the United States, #) is a unit of mass in a number of different systems, including various systems of units of mass that formed part of English units, Imperial units, and United States customary units. Its size can vary from system to system. The most commonly used pound today is the international avoirdupois pound.
The distinction between mass and weight (or force), and its development, is discussed in the article on weight. In some circumstances, the pound is used as the name of a unit of force. That usage is discussed in the article on pound-force (a unit of force based on a mass of one (avoirdupois) pound and the force of gravity at the surface of the Earth).
The international avoirdupois pound is equal to exactly 453.59237 grams. The definition of the international pound was agreed by the United States and countries of the Commonwealth of Nations in 1958.
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.
Equivalence to other units of mass
The table below sets out the relationships between the avoirdupois pound and:
- the troy pound (see below);
- three other historical pounds (see below): the Tower pound, the merchant pound and the London pound;
- the 500-gram metric pound used in some places for some time during metrication (see below); and
- an International System of Units (SI) unit of mass, the gram.
The pound as a name for a unit of mass (or weight, before the distinction between mass and weight developed) has a long history. The history of the pound goes hand in hand with the history of the related systems of units of mass and their applications.
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 of mass similar to a pound. This is the origin of the abbreviation “lb” or “Template:Unicode” for the pound. The “s” at the end of “lbs” simply denotes the plural form.
In the United Kingdom there is a historical link between the pound as a unit of mass and the pound as a unit of currency (the pound sterling), because the unit of currency was defined in the past in terms of a specific quantity of silver.
The avoirdupois pound was invented by London merchants in 1303.
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 jewelers.
Prior to a change during the reign of Henry VIII of England (see below), the avoirdupois pound was based on independent standards which had been measured as about 7,002 troy grains.
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.
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 very slightly different value (in relation to the kilogram) than it does now, of approximately 0.453592338 kg. (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 the United States, the (avoirdupois) pound as a unit of mass has been officially defined in terms of the kilogram since 1893. In 1893, the relationship was specified to be 2.20462 pounds were equal to 1 kilogram. In 1894, the relationship was specified to be 2.20462234 pounds were equal to 1 kilogram. This change followed a determination of the British pound. The current international pound differs from the United States 1894 pound by approximately one part in 10 million.
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 different standards of mass (weight). Some of these other pounds are described below.
Roman libra or pound
A Roman libra or pound is an ancient unit of mass that was equivalent to approximately 327 grams. It was divided into 12 uncia, or ounces.
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, and an international troy pound is 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 pound 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:
- Encyclopædia Britannica (a U.S. encyclopedia for about a century now) which uses either avoirdupois pounds or troy ounces, likely never both in the same article (which would make an awkward system with 14 7/12 ounces to a pound), and
- the mass of King Tut's sarcophagus lid. This is 110 kilograms. It is often stated to have been 242 or 243 (avoirdupois) pounds but sometimes, much less commonly, it is stated as 296 (troy) pounds.
French livre or pound
The livre (translated as the pound), is a French name for various units of mass since the Middle Ages. The names continues to be used today to refer to a metric pound (see below).
The livre poids de marc or livre de Paris was equivalent to about 7,555 grains or about 489.5 grams and was used between the 1350s and the late 18th century.. It was introduced by the government of King John II of France.
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 (see above).
A Tower pound was equal to 5,400 grains. Prior to 1528 the British monetary unit also known as the pound was a Tower pound of silver (worth about £38 today). In 1528, the standard was changed to the Troy pound.
Libra mercatoria or mercantile, merchants' or commercial pound
A mercantile pound or libra mercantoria, also known as a merchants' pound or commercial pound, is an obsolete unit of mass used in England for most goods (other than money, spices and electuaries) until a point during the 14th century. It was equal to 9,600 wheat grains (equivalent to 6,750 grains). There were 12 tower ounces in a tower pound, and a merchant pound was 15 tower ounces.
London or mercantile pound
A London pound was equal to 7,200 grains. A London pound was 16 tower ounces or, equivalently, 15 troy ounces.
Scottish or trone pound
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. If the pound is used in the Netherlands today it is likely to refer to 500 grams; the former definition has fallen out of use. However, the 100-gram ounce remains in limited use.
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.
- ↑ Quoted by Laws LJ in  EWHC 195 (Admin). Retrieved on 2006-08-12.
- ↑ United States National Bureau of Standards (1959-06-25). Notices "Refinement of values for the yard and the pound". Retrieved on 2006-08-12.
- ↑ 3.0 3.1 3.2 3.3 Sizes, Inc. (2001-03-16). Pre-metric French units of mass livre and smaller. Retrieved on 2006-08-12.
- ↑ Sizes, Inc. (2003-07-28). Jersey pound. Retrieved on 2006-08-12.
- ↑ Zupko, Ronald (1985-12-01). Dictionary of Weights and Measures for the British Isles: The Middle Ages to the 20th Century. DIANE Publishing. ISDN 087169168X.
- ↑ English Weights & Measures. Retrieved on 2006-08-12.