Officially the pound is the name for at least three different units of mass:
- The pound (avoirdupois).
- The troy pound.
- The metric pound, no longer officially sanctioned but still in informal use in some places.
Standards bodies define the pound as a unit of mass, which are the pounds most people in everyday usage use as a unit of weight.
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 signs £ and ₤ (crossed-out L's) for the currency derived from this. The word "pound" itself comes from the Latin pendere, to weigh, while libra meant "scales, balances".
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
Main article: avoirdupois
The avoirdupois pound was invented by London merchants in 1303.
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 a 1959 agreement among the national standards laboratories of the United States, Canada, the United Kingdom, South Africa, Australia, and New Zealand.  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).
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.
There are 16 ounces in a pound (avoirdupois). This pound is equal to exactly 7000 grains, where a grain is exactly 0.06479891 gram. This relationship between avoirdupois pounds and troy grains has held true since the avoirdupois pound was redefined in terms of the troy units in the reign of Henry VIII, abandoning independent standards which had been measured as about 7002 grains troy. Since then, the grain has often been considered as a part of the avoirdupois system as well, even though it does not fit very well in that system.
Main article: Troy weight
A troy pound is a unit of mass in the United States, Canada, Australia, the United Kingdom, and other places. 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 avoirdupois.
The troy pound is now used only for measurements of precious metals such as gold, silver, and platinum, and sometimes gems such as opals. Most weight measurements of precious metals using pounds and ounces use troy pounds and ounces, even though it is not always explicitly stated that this is the case. Some notable exceptions are Encyclopędia 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 a weird system with 14 7/12 ounces to a pound), and King Tut's sarcophagus lid, which is often stated to have been 242 or 243 pounds (avoirdupois; when it is, much less commonly, stated as 296 pounds, then the pounds are troy).
One troy pound = 12 troy ounces = 240 pennyweight = 5760 grains.
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.
Main article: kilogram
In many countries that use the SI or metric system, the pound (or its translation, for example, the German Pfund, the French livre, or the Dutch pond) is used as an informal term for half of a kilogram, therefore for this case the pound is 500 grams. In many cases, this was an official redefinition back in the 19th century, but its use is generally no longer officially sanctioned. These replaced hundreds of older pounds, for example, one of around 459 to 460 grams in Spain, Portugal, and Latin America; 498.1 g in Norway; and several different ones in what is now Germany. In the case of the Dutch pond, this was officially redefined as 1 kg, with an ounce of 100 g; the former has fallen out of use, and if the pound is used today it is likely the 500 g variety, but the 100 g ounce remains in limited use.
Pound as a unit of weight
When a pound is called a "unit of weight", it is usually a unit of mass. This is always true for the term net weight, for example.
Pounds are also used for the force definitions of weight, in which the pound force is a unit of force equal to 4.448 newtons. That is the force due to gravity of a pound (avoirdupois) where the acceleration of gravity is 32.17405 ft/s². The troy units of weight are never units of force. Pounds-force are never used for net weight, nor are they used for body weight in the medicial sciences, in zoology, or in sports. Main article: pound-force
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 systems are no longer used in science (and are 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.
In many contexts, there is a long history of considering the pound to be a unit of mass:
- Pounds were primarily a measure of how much stuff people had, for the purposes of trade. We know what we use for those purposes today--the only pounds legal for trade anywhere in the world are those defined as units of mass exactly equal to 0.45359237 kg.
- Mass-measuring balances were the only weighing instruments anybody ever used before the 19th century.2
- Over time, the various keepers of the standards redefinedpounds in terms of the metric system (which has happened in case of the avoirdupois and troy pounds as well as the metric pounds), they were defined in terms of the kilogram, not the dyne or the newton.
- When units such as the British thermal unit are defined based on the pound, those pounds are units of mass just like the grams or kilograms used as the basis of the definition of calories.
- On labels of products sold in the U.S., the pounds and ounces are units of mass, like the grams and kilograms which appear right alongside them.
On the other hand, pounds are always to be construed as a unit of force in contexts such as these:
Thrust of rocket or jet engines in pounds-force.
Torque in foot-pounds or pound-feet.
Pressure in pounds per square foot or pounds per square inch.
There are some contexts in which the word "weight" is customarily used in a misleading way:
- In commerce, the terms "net weight," troy weight, and carat weight actually refer to mass rather than weight. The pounds used for this purpose are units of mass.
- The scientific terms "molecular weight," "atomic weight," and "formula weight" all actually refer to mass, which is why some now refer to "molecular mass," etc. If a "pound mole" is used, it is based on the pound as a unit of mass.
There are three practical ways of doing calculations with mass and force in the fps systems (and other systems such as inch-pound-second systems not discussed here), which the following table summarizes and compares with the SI.
|unit of time
|unit of distance
|unit of mass and weight
|unit of force and weight
|Newton's second law
||F = ma
||F = ma
||F = ma/gc
||F = ma
|weight of an object
||W = mg
||W = mg
||W = mg/gc
||W = mg
The absolute and gravitational fps systems are coherent systems of units which both share with the SI the advantage of avoiding needless complication in several of the formulas used, whereas the engineering fps system requires the introduction of the factor gc, which is a dimensionless constant, defined as 32.17405 lb ft/(lbf s2) and approximately equal to the typical acceleration of gravity on Earth, in ft/s2. This must be distinguished from the actual local value of g.
No one of the three fps systems is more correct than the other two. None of our ordinary measurements are made in the context of any of these specialized subsets of mechanical units, used only in calculations.
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. When giving data to be used in calculations, it is not a good idea to use the term pound without clarifying whether mass or force is intended. If force is what is meant, the symbol "lbf" or the term "pounds-force" can be used for clarification. For mass, one can specify "pounds-mass."
It is primarily the United States which uses them today, though other places such as livestock markets in Canada do still sell cattle in cents per pound, even though they sell hogs in cents per kilogram (actually, it is most often quoted as dollars per hundred pounds and dollars per hundred kilograms).
Note 2: Copies of the standard pound in London were distributed to other locations, where they served as the local standard. These copies have the same mass at the new location, even though they exert a (slightly) different amount of force due to regional variations in the Earth's gravitational field. The standards for pounds always were standards of mass, not standards of force.