# Weight Conversions

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Kilogram - The kilogram or kilogramme (symbol: kg) is the base unit of mass in the International System of Units (SI, from the French Le Systeme International d'Unites). The kilogram is defined as being equal to the mass of the International Prototype Kilogram (IPK), which is almost exactly equal to the mass of one liter of water. It is the only SI base unit with an SI prefix as part of its name. It is also the only SI unit that is still defined in relation to an artifact rather than to a fundamental physical property that can be reproduced in different laboratories.

In everyday usage, the mass of an object, which is measured in kilograms, is often referred to as its weight. However, the term weight in strict scientific contexts refers to the gravitational force of an object. Throughout most of the world, force is measured with the SI unit newton and the non-SI unit kilogram-force. Similarly, the avoirdupois (or international) pound, used in both the Imperial system and U.S. customary units, is a unit of mass and its related unit of force is the pound-force. The avoirdupois pound is defined as exactly 0.45359237 kg, making one kilogram approximately equal to 2.2046 avoirdupois pounds.

Many units in the SI system are defined relative to the kilogram so its stability is important. After the International Prototype Kilogram had been found to vary in mass over time, the International Committee for Weights and Measures (known also by its French-language initials CIPM) recommended in 2005 that the kilogram be redefined in terms of a fundamental constant of nature. No final decision is expected before 2011.

Gram - The gram (often gramme in British English), (Greek/Latin root gramma); symbol g, is a unit of mass. Originally defined as "the absolute weight of a volume of pure water equal to the cube of the hundredth part of a metre, and at the temperature of melting ice" (later 4 °C), a gram is now defined as one one-thousandth of the SI base unit, the kilogram, or 1x10-3 kg, which itself is defined as being equal to the mass of a physical prototype preserved by the International Bureau of Weights and Measures.

The International System of Units abbreviation for the gram is g, and follows the numeric value with a space, as in "640 g". In some fields and regions, the international standard units for units are used quite strictly, in particular in technical and scientific publications and in legally regulated product labels. In other contexts, a wide range of other unofficial abbreviations have been encountered, such as gr, gm, grm, gms, grms. The use of abbreviations such as "gm", "Gm", or "GM" for grams could potentially lead to serious errors in healthcare settings where accidentally transposing "gm" to "mg" (milligrams) would result in a 1000 times dosage difference. It would therefore be prudent to use "g" as the abbreviation for grams in any healthcare setting.

Pound - 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 kilogram. The word pound comes from the Latin word pondus meaning "weight". The abbreviation lb comes from the Latin word libra, meaning "scales, balances", which also described a Roman unit similar to the pound.

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.

British pounds - 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. Historically the pound sterling was a Tower pound of silver (worth about 56 pounds or about \$92 US today). In 1528 the standard was changed to the Troy pound (worth about 60 pounds or \$100).

Avoirdupois pound - 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.

International pound - 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. 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 0.45359237 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.

Metric pounds - 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. 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.

Milligram - A milligram, abbreviated mg, is 1 thousandth (.001) of a gram.

Troy Ounce - The troy ounce (ozt) is 480 grains, somewhat heavier than an avoirdupois ounce (437.5 grains). A grain is 64.79891 mg; hence one troy ounce is 31.1034768 grams, about 10 percent more than the avoirdupois ounce, which is 28.349523125 g. The troy ounce is traditionally used over the avoirdupois ounce in the pricing of precious metals, gold, platinum, and silver. The grain, which is identical in both the troy and avoirdupois systems, is used to measure arrow and arrowhead weights in archery, and bullets and powder weights in ballistics. The grain is also common to the apothecaries' system and was long used in medicine, but has been largely replaced by milligrams.

The "troy ounce" is part of the system of troy weights derived from the Roman monetary system. The Romans used bronze bars of varying weights as currency. An "aes grave" weighed in at 1 pound. One twelfth of an aes grave was called an uncia, in English "ounce". Later standardizations would change the ounce to 1/16th of a pound, but the 1/12th pound ounce is still used for measuring precious metals.

Troy Pound - The troy pound (troy) is 5,760 gr (approx. 373.24 g, 12 ozt), while an avoirdupois pound is heavier at 7,000 gr (approx. 453.59 g). There are 12 troy ounces per troy pound, rather than 16 avoirdupois ounces (oz) in the avoirdupois pound (lb) as in the more common avoirdupois system. The avoirdupois pound is 147/12 (approx. 14.583) troy ounces.

Long Ton - Long ton (weight ton or imperial ton) is the name for the unit called the "ton" in the avoirdupois or Imperial system of measurements, as used in the United Kingdom and several other Commonwealth countries. It has been mostly replaced by the short ton in the United States and the tonne (metric ton) elsewhere. It is equal to 2,240 pounds (1,016.0469088 kg) or 35 cubic feet (0.99108963 m3) of salt water with a density of 1.025 g/ml. It has some limited use in the United States, most commonly in measuring the displacement of ships, and was the unit prescribed for warships (e.g. battleships limited to 35,000 long tons (35,561.6 t)) in the international agreements between the world wars.

Ounce - The ounce (abbreviated: oz, the old Italian word onza, now spelled oncia;) is a unit of mass in a number of different systems, including various systems of mass that form part of the imperial and United States customary systems. Its size can vary from system to system. The most commonly used ounces today are the international avoirdupois ounce and the international troy ounce.

Historically, in different parts of the world, at different points in time, and for different applications, the ounce (or its translation) has referred to broadly similar but different standards of mass (or weight, before the distinction between weight and mass developed). An ounce is more often a measure of force as opposed to mass or weight, comparable to lbf, or pound force.

International avoirdupois ounce - The avoirdupois ounce is the most commonly used ounce today. It is defined to be one sixteenth of an avoirdupois pound. It is therefore equal to 437.5 grains. In 1958 the United States and countries of the Commonwealth of Nations agreed to define the international avoirdupois pound to be exactly 0.45359237 kilograms. Consequently, since 1958, the international avoirdupois ounce is exactly 28.349523125 grams by definition. The ounce is commonly used as a unit of mass in the United States. On January 1, 2000 it ceased to be a legal unit of measure within the United Kingdom for economic, health, safety or administrative purposes but remains a familiar unit, especially amongst older people.

International troy ounce - A troy ounce (abbreviated as t oz) is equal to 480 grains. Consequently, the international troy ounce is equal to exactly 31.1034768 grams. There are 12 troy ounces in the now obsolete troy pound. Today, the troy ounce is used only to express the mass of precious metals such as gold, platinum, palladium or silver. For historical measurement of gold, a fine ounce is a troy ounce of 99.5% (".995") pure gold, a standard ounce is a troy ounce of 22 carat gold, 91.66% pure (11 "fine ounces" plus one ounce of alloy material), in modern day, an ounce of gold (1 troy ounce) is referred as a 99.99% pure gold piece or gold grains (gold shot).

Apothecaries' ounce - The obsolete apothecaries' ounce equivalent to the troy ounce, was formerly used by apothecaries (now called pharmacists or chemists).

Maria Theresa ounce - "Maria Theresa ounce" was once introduced in Ethiopia and some European countries, which was equal to the weight of one Maria Theresa thaler, or 28.0668 g. Both the weight and the value are the definition of one "Birr", still in use in present-day Ethiopia and formerly in Eritrea.

Metric ounces - Some countries have redefined their ounces to fit in with the metric system. The Dutch have redefined their ounce (in Dutch, ons) as 100 grams. The Dutch's metric values, such as 1 ons = 100 grams, is inherited, adopted and taught in Indonesia since elementary school. It is also formally written in Indonesian national dictionary (Kamus Besar Bahasa Indonesia) and elementary school's formal manual book. East Asia has a traditional ounce, known as a tael, of varying value. In China, it has been given a metric value of 50 grams.

Metric Ton - A tonne (t) or metric ton (U.S.), also referred to as a metric tonne, is a measurement of mass equal to 1,000 kg or 2,204.62262 lb, or approximately the mass of one cubic metre of water. It is not an SI unit but is accepted for use with the SI. If prefixes were used completely consistently, the SI unit for a tonne would be a megagram, or Mg (see SI prefix), but this term is rarely used. The spelling tonne pre-dates the introduction of the SI system in 1960 (it has been used in France for about two and a half centuries), and is now used as the standard spelling for the metric mass measurement in most English-speaking countries. In the U.S., the units were originally referred to using the French words millier or tonneau, but these terms are now obsolete. The Imperial and US customary units comparable to the tonne are both spelled ton in English.

Grain - In many cultures, a grain is a unit of measurement of mass that is based upon the mass of a single seed of a typical cereal. Historically, in Europe, the average masses of wheat and barley grain were used to define units of mass. Since 1958, the grain or troy grain (Symbol: gr) measure has been redefined on the basis of the unit of mass of the International System of Units as precisely 64.79891 milligrams. Thus, there are precisely 7,000 grains per avoirdupois pound in the Imperial and U.S. customary units. In fact, the grain is the only unit of mass measure common to the traditional three English mass and weight systems (avoirdupois, Apothecaries', troy). Moreover, the measure for pearls and diamondsâ€”the pearl grain and the metric grain-are equal to 1/4 of a (metric) carat, i.e. 50 mg (0.77 gr).

Stone - The stone is a unit of weight, that is, the force the gravitational field of the Earth exerts on a mass. The Imperial unit of mass is the slug, which weighs 32.17405 pounds -- or 2.29815 stone -- in Earth's gravitational field. The stone is part of the Imperial system of weights and measures used in the United Kingdom, and formerly used in several Commonwealth countries. It is equal to 14 pounds (more precisely avoirdupois pounds), or approximately 6.35 kilograms. Eight stone make a hundredweight in the Imperial system. When used as the unit of measurement, the plural form of stone is correctly stone (as in, "11 stone"), though stones is sometimes used, but not usually by British natives. The abbreviation is st. When describing the units, the correct plural is stones (as in, "Please enter your weight in stones and pounds").

The stone was originally used for weighing agricultural commodities. Historically the number of pounds in a stone varied by commodity, and was not the same in all times and places even for one commodity. Potatoes, for example, were traditionally sold in stone and half-stone (14-pound and 7-pound) quantities but the OED contains examples. Another example is the definition of the "stone" in the 1772 edition of Encyclopaedia Britannica which reads "STONE also denotes a certain quantity or weight of some commodities. A stone of beef, in London, is the quantity of eight pounds; in Hertfordshire, twelve pounds; in Scotland sixteen pounds."

Although the 1985 Weights and Measures Act expressly prohibited the use of the stone as a unit of measure for purposes of trade (other than as a supplementary unit), the stone remains widely used within the United Kingdom (and Ireland) as a means of expressing human body weight. People in these countries normally describe themselves as weighing, for example, "11 stone 4" (11 stone and 4 pounds), rather than "72 kilograms" in most other countries, or "158 pounds" (the conventional way of expressing the same weight in the United States and Canada). Its widespread colloquial use may be compared to the persistence in the United Kingdom of other Imperial units like the foot, the inch, and the mile, despite these having been supplanted entirely or partly by metric units in official use and other contexts. Thus on a National Health Service Web site the user may select either metric or Imperial units, but the law requires that if this information is officially recorded, then such records shall be in metric units. Outside the United Kingdom, stone may also be used to express body weight in casual contexts in other Commonwealth countries.

Short Ton - The short ton (S/T) is a unit of weight equal to 2,000 pounds (907.18474 kg). In the United States it is often called simply ton without distinguishing it from the metric ton (or tonne, 1,000 kilograms) or the long ton (2,240 pounds (1,020 kg)); rather, the other two are specifically noted. There are, however, some U.S. applications for which unspecified tons normally means long tons (for example, Navy ships) or metric tons (world grain production figures).

Both the long and short ton are defined as 20 hundredweights, but a hundredweight is 112 pounds (50.8 kg) (which is equal to 8 stone, 1 stone/6.35 kilograms) in the Imperial system (long or gross hundredweight) and 100 pounds (45.4 kg) in the U.S. system (short or net hundredweight).

The spelling tonne is from Gallic and French. The term applied to the barrel of the largest size. In Old English the spelling was tunne, "cask". A full cask about a metre high could easily weigh a metric tonne, since the volume of the antiquated British wine cask tun is defined as 954 litres which for water (density = 1 g/cm3) amounts to as many kilograms. In the context of heraldry, the term 'tun' is also recognized - again meaning a very large barrel, such as a brewer might use. For example, tuns are part of the blazon of the brewer's guild of London, England.