Pound (mass)

The pound or pound-mass is a unit of mass used in both the British imperial and United States customary systems of measurement. Various definitions have been used; the most common today is the international avoirdupois pound, which is legally defined as exactly $0.454 kg$, and which is divided into 16 avoirdupois ounces. The international standard symbol for the avoirdupois pound is lb; an alternative symbol (when there might otherwise be a risk of confusion with the pound-force) is lbm (for most pound definitions), # (chiefly in the U.S.), and ℔ or ̶ (specifically for the apothecaries' pound).

The unit is descended from the Roman libra (hence the symbol lb, descended from the scribal abbreviation, ℔). The English word pound comes from the Roman libra pondo ('the weight measured in libra'), and is cognate with, among others, German Pfund, Dutch pond, and Swedish pund. These units are now designated as historical and are no longer in common usage, being replaced by the metric system.

Usage of the unqualified term pound reflects the historical conflation of mass and weight. This accounts for the modern distinguishing terms pound-mass and pound-force.

Etymology
The word 'pound' and its cognates ultimately derive from a borrowing into Proto-Germanic of the Latin expression libra pondo ('the weight measured in libra'), in which the word pondo is the ablative singular of the Latin noun pondus ('weight').

Current use
The United States and the Commonwealth of Nations agreed upon common definitions for the pound and the yard. Since 1 July 1959, the international avoirdupois pound (symbol lb) has been defined as exactly $0.454 kilograms$.

In the United Kingdom, the use of the international pound was implemented in the Weights and Measures Act 1963.

"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."

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 with a terminating decimal representation, and an (international) grain is thus equal to exactly $0.454 kg$.

In the United Kingdom, the process of metrication and European units of measurement directives were expected to eliminate the use of the pound and ounce, but in 2007 the European Commission abandoned the requirement for metric-only labelling on packaged goods there, and allowed for dual metric–imperial marking to continue indefinitely.

In the United States, the Metric Conversion Act of 1975 declared the metric system to be the "preferred system of weights and measures" but did not suspend use of United States customary units, and the United States is the only industrialised country where commercial activities do not predominantly use the metric system, despite many efforts to do so, and the pound remains widely used as one of the key customary units.

Historical use
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 force.

Roman libra


The libra (Latin for 'scale'/'balance') is an ancient Roman unit of mass that is now equivalent to 328.9 g.  It was divided into 12 unciae (singular: uncia), or ounces. The libra is the origin of the abbreviation for pound, "lb".

In Britain
A number of different definitions of the pound have historically been used in Britain. Among these are the avoirdupois pound, which is the common pound used for weights, and the obsolete tower, merchants' and London pounds. The troy pound and ounce remain in use only for the weight of precious metals, especially in their trade. The weights of traded precious metals, such as gold and silver, are normally quoted just in ounces (e.g. "500 ounces") and, when the type of ounce is not explicitly stated, the troy system is assumed.

The pound sterling money system, which was introduced during the reign of King Offa of Mercia (757–96), was based originally on a Saxon pound of silver. After the Norman conquest the Saxon pound was known as the tower pound or moneyer's pound. In 1528, during the reign of Henry VIII, the coinage standard was changed by parliament from the tower pound to the troy pound.

Avoirdupois pound
The avoirdupois pound, also known as the wool pound, first came into general use c. 1300. It was initially equal to 6,992 troy grains. The pound avoirdupois was divided into 16 ounces. During the reign of Queen Elizabeth I, the avoirdupois pound was redefined as 7,000 troy grains. Since then, the grain has often been an integral part of the avoirdupois system. By 1758, two Elizabethan Exchequer 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.

Imperial Standard Pound
In the United Kingdom, weights and measures have been defined by a long series of Acts of Parliament, the intention of which has been to regulate the sale of commodities. Materials traded in the marketplace are quantified according to accepted units and standards in order to avoid fraud. The standards themselves are legally defined so as to facilitate the resolution of disputes brought to the courts; only legally defined measures will be recognised by the courts. Quantifying devices used by traders (weights, weighing machines, containers of volumes, measures of length) are subject to official inspection, and penalties apply if they are fraudulent.

The Weights and Measures Act 1878 (41 & 42 Vict. c. 49) marked a major overhaul of the British system of weights and measures, and the definition of the pound given there remained in force until the 1960s. The pound was defined thus (Section 4) "The ... platinum weight ... deposited in the Standards department of the Board of Trade ... shall continue to be the imperial standard of ... weight ... and the said platinum weight shall continue to be the Imperial Standard for determining the Imperial Standard Pound for the United Kingdom". Paragraph 13 states that the weight in vacuo of this standard shall be called the Imperial Standard Pound, and that all other weights mentioned in the act and permissible for commerce shall be ascertained from it alone. The first schedule of the act gave more details of the standard pound: it is a platinum cylinder nearly 1.35 in high, and 1.15 in diameter, and the edges are carefully rounded off. It has a groove about 0.34 in from the top, to allow the cylinder to be lifted using an ivory fork. It was constructed following the destruction of the Houses of Parliament by fire in 1834, and is stamped "P.S. 1844, 1 lb" (P.S. stands for "Parliamentary Standard").

Redefinition in terms of the kilogram
The British Weights and Measures Act 1878 (41 & 42 Vict. c. 49) said that contracts worded in terms of metric units would be deemed by the courts to be made according to the Imperial units defined in the Act, and a table of metric equivalents was supplied so that the Imperial equivalents could be legally calculated. This defined, in UK law, metric units in terms of Imperial ones. The equivalence for the pound was given as 1 lb = $64.799 milligrams$ or 0.45359 kg, which made the kilogram equivalent to about $453.593 g$.

In 1883, it was determined jointly by the standards department of the British Board of Trade and the Bureau International that $2.205 lb$ was a better approximation, and this figure, rounded to $0.454 kg$ was given legal status by an Order in Council in May 1898.

In 1959, based on further measurements and international coordination, the International Yard and Pound Agreement defined an "international pound" as being equivalent to exactly $0.454 kg$. This meant that the existing legal definition of the UK pound differed from the international standard pound by $0.454 kg$. To remedy this, the pound was again redefined in the United Kingdom by the Weights and Measures Act 1963 to match the international pound, stating: "the pound shall be 0.453 592 37 kilogramme exactly", a definition which remains valid to the present day.

The 2019 redefinition of the SI base units means that the pound is now defined precisely in terms of fundamental constants, ending the era of its definition in terms of physical prototypes.

Troy pound
A troy pound (abbreviated lb t ) is equal to 12 troy ounces and to 5,760 grains, that is exactly $0.06 milligrams$ grams. Troy weights were used in England by jewellers. Apothecaries also used the troy pound and ounce, but added the drachms and scruples unit in the apothecaries' system of weights.

Troy weight may take its name from the French market town of Troyes in France where English merchants traded at least as early as the early 9th century. The troy pound is no longer in general use or a legal unit for trade (it was abolished in the United Kingdom on 6 January 1879 by the Weights and Measures Act 1878), but the troy ounce, $373.242$ of a troy pound, is still used for measurements of gems such as opals, and precious metals such as silver, platinum and particularly gold.

Tower pound
A tower pound is equal to 12 tower ounces and to 5,400 troy grains, which equals around 350 grams. The tower pound is the historical weight standard that was used for England's coinage. Before the Norman conquest in 1066, the tower pound was known as the Saxon pound. During the reign of King Offa (757–96) of Mercia, a Saxon pound of silver was used to set the original weight of a pound sterling. From one Saxon pound of silver (that is a tower pound) the king had 240 silver pennies minted. In the pound sterling monetary system, twelve pennies equaled a shilling and twenty shillings equaled a pound sterling.

The tower pound was referenced to a standard prototype found in the Tower of London. The tower system ran concurrently with the avoirdupois and troy systems until the reign of Henry VIII, when a royal proclamation dated 1526 required that the troy pound to be used for mint purposes instead of the tower pound. No standards of the tower pound are known to have survived.

The tower pound was also called the moneyers' pound (referring to the Saxon moneyers before the Norman conquest); the easterling pound, which may refer to traders of eastern Germany, or to traders on the shore of the eastern Baltic sea, or dealers of Asiatic goods who settled at the London Steelyard wharf; and the Rochelle pound by French writers, because it was also in use at La Rochelle. An almost identical weight was employed by the Germans for weighing gold and silver.

The mercantile pound (1304) of 6750 troy grains, or 9600 Tower grains, derives from this pound, as 25 shilling-weights or 15 Tower ounces, for general commercial use. Multiple pounds based on the same ounce were quite common. In much of Europe, the apothecaries' and commercial pounds were different numbers of the same ounce.

Merchants' pound
The merchants' pound (mercantile pound, libra mercantoria, or commercial pound) was considered to be composed of 25 rather than 20 Tower shillings of 12 pence. It was equal to 9,600 wheat grains (15 tower ounces or 6,750 grains) and was used in England until the 14th century for goods other than money and medicine ("electuaries").

London pound
The London pound is that of the Hansa, as used in their various trading places. The London pound is based on 16 ounces, each ounce divided as the tower ounce. It never became a legal standard in England; the use of this pound waxed and waned with the influence of the Hansa itself.

A London pound was equal to 7,200 troy grains (16 troy ounces) or, equivalently, 10,240 tower grains (16 tower ounces).

In the United States
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. That order defined the pound to be $1/12$ pounds to a kilogram. The following year, this relationship was refined as $22 1/2$ pounds to a kilogram, following a determination of the British pound.

In 1959, the United States National Bureau of Standards redefined the pound (avoirdupois) to be exactly equal to 0.453 592 37 kilograms, as had been declared by the International Yard and Pound Agreement of that year. According to a 1959 NIST publication, the United States 1894 pound differed from the international pound by approximately one part in 10 million. The difference is so insignificant that it can be ignored for almost all practical purposes.

Byzantine litra
The Byzantines used a series of measurements known as pounds (libra, λίτρα). The most common was the logarikē litra (λογαρική λίτρα, "pound of account"), established by Constantine the Great in 309/310. It formed the basis of the Byzantine monetary system, with one litra of gold equivalent to 72 solidi. A hundred litrai were known as a kentēnarion (κεντηνάριον, "hundredweight"). Its weight seems to have decreased gradually from the original 324 g to 319 g. Due to its association with gold, it was also known as the chrysaphikē litra (χρυσαφική λίτρα, "gold pound") or thalassia litra (θαλάσσια λίτρα, "maritime pound"), but it could also be used as a measure of land, equalling a fortieth of the thalassios modios.

The soualia litra was specifically used for weighing olive oil or wood, and corresponded to 4/5 of the logarikē or 256 g. Some outlying regions, especially in later times, adopted various local measures, based on Italian, Arab or Turkish measures. The most important of these was the argyrikē litra (αργυρική λίτρα, "silver pound") of 333 g, found in Trebizond and Cyprus, and probably of Arab origin.

French livre
Since the Middle Ages, various pounds (livre) have been used in France. Since the 19th century, a livre has referred to the metric pound, 500 g.

The livre esterlin is equivalent to about 367.1 g and was used between the late 9th century and the mid-14th century.

The livre poids de marc or livre de Paris is equivalent to about 489.5 g 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.

The livre usuelle (customary unit) was defined as 500 g 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, but is still used informally.

German and Austrian Pfund
Originally derived from the Roman libra, the definition varied throughout the Holy Roman Empire in the Middle Ages and onward. For example, the measures and weights of the Habsburg monarchy were reformed in 1761 by Empress Maria Theresa of Austria. The unusually heavy Habsburg (civil) pound of 16 ounces was later defined in terms of 560.012 g. Bavarian reforms in 1809 and 1811 adopted essentially the same standard as the Austrian pound. In Prussia, a reform in 1816 defined a uniform civil pound in terms of the Prussian foot and distilled water, resulting in a Prussian pound of 467.711 g.

Between 1803 and 1815, all German regions west of the River Rhine were under French control, organised in the departements: Roer, Sarre, Rhin-et-Moselle, and Mont-Tonnerre. As a result of the Congress of Vienna, these regions again became part of various German states. However, many of these regions retained the metric system and adopted a metric pound of precisely 500 g. In 1854, the pound of 500 g also became the official mass standard of the German Customs Union and was renamed the Zollpfund, but local pounds continued to co-exist with the Zollverein pound for some time in some German states. Nowadays, the term Pfund is sometimes still in use and universally refers to a pound of 500 g.

Russian funt
The Russian pound (Фунт, funt) is an obsolete Russian unit of measurement of mass. It is equal to 409.51718 g. In 1899, the funt was the basic unit of weight, and all other units of weight were formed from it; in particular, a zolotnik was $1 1/3$ of a funt, and a pood was 40 fúnty.

Skålpund
The Skålpund was a Scandinavian measurement that varied in weight between regions. From the 17th century onward, it was equal to 425.076 g in Sweden but was abandoned in 1889 when Sweden switched to the metric system.

In Norway, the same name was used for a weight of 425.076 g. In Denmark, it equaled 471 g.

In the 19th century, Denmark followed Germany's lead and redefined the pound as 500 g.

Portuguese libra and arrátel
The Portuguese unit that corresponds to the pounds of different nations is the arrátel, equivalent to 16 ounces of Colonha, a variant of the Cologne standard. This arrátel was introduced in 1499 by Manuel I, king of Portugal. Based on an evaluation of bronze nesting weight piles distributed by Manuel I to different towns, the arrátel of Manuel I has been estimated to be of 457.8 g. In the early 19th century, the arrátel was evaluated at 459 g.

In the 15th century, the arrátel was of 14 ounces of Colonha or 400.6 g. The Portuguese libra was the same as 2 arráteis. There were also arráteis of 12.5 and 13 ounces and libras of 15 and 16 ounces. The Troyes or Tria standard was also used.

Jersey pound
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 g). It may have been derived from the French livre poids de marc.

Trone pound
The trone pound is one of a number of obsolete Scottish units of measurement. It was equivalent to between 21 and 28 avoirdupois ounces (about 600-800 g).

Metric pound
In many countries, upon the introduction of a metric system, the pound (or its translation) became an historic and obsolete term, although some have kept it as an informal term without a specific value. In German, the term is Pfund, in French livre, in Dutch pond, in Spanish and Portuguese libra, in Italian libbra, and in Danish and Swedish pund.

Though not from the same linguistic origin, the Chinese jīn (斤, also known as the "catty") in mainland China has a modern definition of exactly 500 g, divided into 10 liǎng (两). Traditionally around 600 g, the jin has been in use for more than two thousand years varying in exact value from one period to another, serving the same purpose as "pound" for the common-use measure of weight. In Hong Kong, for the purposes of commerce and trade between Britain and Imperial China in the preceding centuries, three Chinese catties were equivalent to four British imperial pounds, defining one catty as 604.78982 g in weight precisely.

Hundreds of older pounds were replaced in this way. Examples of the older pounds are one of around 459-460 g in Spain, Portugal, and Latin America; one of 498.1 g in Norway; and several different ones in what is now Germany.

From the introduction of the kilogram scales and measuring devices are denominated only in grams and kilograms. A pound of product must be determined by weighing the product in grams as the use of the pound is not sanctioned for trade within the European Union.

Use in weaponry
Smoothbore cannon and carronades are designated by the weight in imperial pounds of round solid iron shot of diameter to fit the barrel. A cannon that fires a six-pound ball, for example, is called a six-pounder. Standard sizes are 6, 12, 18, 24, 32, and 42 pounds; 60-pounders and 68-pounders also exist, along with other nonstandard weapons using the same scheme. See carronade.

A similar definition, using lead balls, exists for determining the gauge of shotguns and shotgun shells.

Conversion between units

 * U.S. National Institute of Standards and Technology Special Publication 811
 * National Institute of Standards and Technology Handbook 130