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Officially the pound is the name for at least three different units of mass:

• the pound (avoirdupois)
• the troy pound
• the obsolete imperial pound

There also exists an unofficial metric pound.

While most standards bodies define the pound as a unit of mass, most people in everyday usage use the pound as a unit of weight.

Origins

The Latin word libra describes a Roman unit of weight similar to a pound, and the abbreviation "lb" for the unit of weight and the sign £ (a crossed-out L) for the currency derived from this. The word "pound" itself comes from the Latin pendere, to weigh.

Measurement systems

In the Imperial system (often referred to as the pound-inch system, or the British system in the United States) there are two basic pounds defined, and also an obsolete definition of one variant of the pound:

Pound (avoirdupois) or international pound

The pound (avoirdupois) or international pound, abbreviation "lb" or sometimes # in the United States, is the mass unit defined as exactly 0.45359237 kilograms (or 453.59237 grams). This definition has been in effect since 1959 in the United States. It is part of the avoirdupois system of mass units.

In the United States, the pound has been officially defined as a unit of mass and defined in relation to the kilogram since 1893, but its value in relation to the kilogram was altered slightly in 1894, and again to its current value in 1959 (which only differs from the 1894 definition by approximately one part in 10 million).

There are 16 ounces in a pound (avoirdupois). The pound is equal to exactly 7000 grains, where a grain is exactly 0.06479891 gram. The legal definition of the pound in the United Kingdom and Canada are the same as in the United States, and were unified to their current value in 1960.

Imperial pound

In the United Kingdom, the pound was similarly 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 old value is sometimes called the imperial pound, and this definition and terminology are obsolete unless referring to the slightly-different 1878 definition.

Troy pound

A pound (troy) is a unit of mass in the United States, Canada, and United Kingdom. The troy pound is a unit of mass equalling exactly 0.3732417216 kilograms. There are 12 troy ounces in a troy pound. A troy pound is equal to exactly 5760 grains, making a troy pound equal to exactly 144/175 pounds. It is part of the troy system of mass units.

The troy pound is used for measurements of precious metals such as gold, silver, and platinum. Any weight measurements of precious metals always uses troy pounds and ounces, even though it is not always explicitly stated that this is the case.

One troy pound = 12 troy ounces = 240 pennyweight.

A pennyweight was literally the weight of a penny, as adopted by King Henry II (1154–1189). This was a sterling silver penny weighing 1/240 of a troy pound.

Metric pound

In many countries that use the SI or metric system the pound (or its translation, e.g. the German Pfund, the French livre or Dutch pond) is used as an unofficial term for half of a kilogram, therefore for this case the pound is 500 grams.

Pound as a unit of weight

While not in accordance with standards bodies, the typical usage of pound in the United States, the pound is implied as a unit of weight equal to 4.448 Newtons. Which is the weight of a pound (avoirdupois) where the acceleration of gravity is 32.17405 ft/s

Which one is meant?

If neither "avoirdupois" nor "troy" is specified, the international pound (avoirdupois) is meant and is by law the only proper definition in the United States, United Kingdom, and Canada; the troy pound has been officially abandoned in the United Kingdom. The valuation of precious metals on U.S. exchanges is specified as dollars per troy ounce, although the fact that the troy ounce is used is usually implied. In the context of vegetable and meat sales within metric countries, a metric pound (500 g) is usually implied.

Force, weight, and mass

Historically, the pound predates the understanding of the distinction between force and mass. Once that distinction became clear, it was natural to ask whether the pound should be construed as a unit of mass, or a unit of force (and weight, which is defined as the gravitational force acting on an object). But because the foot-pound-second system is no longer used in science (and is gradually approaching extinction even in U.S. engineering work), many scientists today would be as bemused by this question as by the question of whether the shekel is a unit of mass or of force. There are two practical ways of doing calculations with mass and force in the fps system, which the following table summarizes and compares with the SI.

The difference between the two variants of the fps system is that the first one uses the slug as its unit of mass, while the second one uses the pound. The former shares with the SI the advantage of avoiding needless complication, whereas the latter requires the introduction of the factor g_s, which is a standard reference value for the gravitational acceleration of 32.17405 ft/s, and must be distinguished from the actual local value of g. Neither system is more correct than the other, but it is necessary when using the fps system to define carefully whether the symbol m is meant to be the mass in units of pounds, or in units of slugs. Both systems use as their unit of force the pound-force (lbf), which is defined as a mass of one pound, multiplied by g_s.

Although the U.S. National Bureau of Standards has defined the pound as a unit of mass, and the pound-force as a unit of force, this distinction is not widely recognized among working physicists, because the fps system has not been used in physics, even in the U.S., since the early 20th century. Therefore it is unwise to use the term pound as a unit of mass, without clarification, and assume that it will be understood as such; it is likely instead to be taken as a unit of force. Among laymen, the pound is used almost exclusively as a unit of weight.

External links

• History of the pound as a unit of mass: U.S. National Institute of Standards and Technology Official Definition, showing history
• Official abbreviations and definitions: U.S. National Institute of Standards and Technology Special Publication 811

• Units of mass
• Imperial units
• US customary units