Pound (unit of mass)

Definitions and common variants

Different systems have used pounds with different sizes. The principal variants encountered in modern and historical contexts include:

• Avoirdupois pound: the internationally agreed standard, defined as exactly 0.45359237 kilograms; it is divided into 16 avoirdupois ounces and is the basis for everyday weights and commerce in countries that use pounds. See also kilogram.
• Troy pound: an older system associated with precious metals; it contains 12 troy ounces and is now rarely used, though the troy ounce remains the common unit for gold, silver and gemstones (one troy ounce ≈ 31.1 grams).
• Apothecaries' pound: historically used in pharmacy and medicine and closely related to the troy system; it is largely obsolete.

History and origin

The concept of a pound traces back to the Roman libra, and many European languages retain a similar word. Over centuries, regional standards and commercial practice produced a variety of "pounds" based on local systems of ounces and grains. In the 20th century international agreements harmonized definitions so that modern avoirdupois pounds used in the United States and by international trade are consistent. For legal and standards background, consult standards.

Uses and examples

Pounds are commonly used to report human body weight, packaged food, and small-scale trade. In the United Kingdom and Ireland people sometimes express body mass in stones and pounds (one stone = 14 pounds). The United States continues to use pounds widely in daily life, commerce and industry. For measurement conversions see conversion.

Notable facts and distinctions

The abbreviation "lb" reflects the historical term rather than the English word "pound." Although the international avoirdupois pound is defined by reference to the kilogram, historical systems such as the troy and apothecaries' pounds survive in specialized contexts. For further reading on weight units and their relations see weight units and measurement history.