Pound (mass)

The pound is a unit of mass equal to exactly 0.45359237 kilograms. It is used chiefly in the United States and in some other customary measurement systems.

International avoirdupois pound: The pound defined by international agreement as exactly 0.45359237 kilograms. This is the precise modern definition of the ordinary pound.
Roman libra: An ancient Roman unit of weight whose name is abbreviated as “lb.” The pound’s abbreviation preserves the name of this historical predecessor.
United States customary units: The measurement system used in the United States, including customary units such as the pound and gallon. The pound remains a standard everyday unit of mass in this system.
Troy pound: A historical unit of mass in the troy system, equal to 12 troy ounces and lighter than the avoirdupois pound. It is a different historical pound, not the ordinary 16-ounce pound.
Kilogram: The SI base unit of mass, defined by fixing the numerical value of the Planck constant. The pound’s exact modern value is expressed in kilograms.
Tower pound: A medieval English pound used chiefly for weighing precious metals, lighter than the later avoirdupois pound. It shows that English pounds once varied by commodity and standard.
Imperial units: A system of units established in Britain in 1824, including the pound and stone. The pound is shared by imperial and US customary measurement, though other units differ.
Metric pound: A nonstandard name sometimes applied to 500 grams, especially in contexts influenced by German measurement. Its rounded metric value differs from the precisely defined international pound.
Ounce (mass): A customary unit of mass equal to one-sixteenth of an avoirdupois pound. It divides the common pound into sixteen smaller units.
Troy weight: A system of mass units traditionally used for precious metals and gemstones. Its distinct pound demonstrates that “pound” has not always named one mass.

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