Why the Symbol of Pound Is lb

Why the Symbol of Pound is lb
The pound unit comes from the old Roman unit libra, which is why it has the symbol lb.

Have you ever wondered why the symbol of pound is lb? The reason is that the unit pound comes from the old Roman unit for mass, the libra pondo. Over time, the Latin phrase got shortened to just the word libra, which meant either weight or the scale of balances used to measure mass. The abbreviation for libra was “lb.” In English, the word pound comes from the pondo part of libra ponda, but the abbreviation still comes from libra.

lb or lbs – Plural of Pound

The proper plural of the pound symbol is a subject of debate because the common usage differs from the technical usage. For scientific and technical publications, the plural of lb is still lb and not lbs. The reason is that the plural of the Latin word libra is librae and not libras. In any case, unit symbols generally lack a plural form. You’ll never see ms for meters or Fs for Fahrenheits, for example. There is a space between the number and the unit (e.g., 2 lb). Some style guidelines require a period after a unit symbol (except for metric symbols), so it is acceptable to write 2 lb., for example.

However, the English language doesn’t always follow the “rules.” In common usage, it’s fine to use lbs, with or without a period. Is it correct? Maybe not, but people will understand your meaning.

Different Types of Pounds

The pound is a unit of mass, although we often use it to measure weight. The true English unit of weight, a force, is the foot-pound (ftâ‹…lb). The amount of matter in a pound varies according to country. The original Roman libra (pound) was around 0.3289 kilograms. The pound could be divided into 12 uncia or ounces. In the United States, the pound is defined as 2.20462234 pounds per metric kilogram, so one pound is 0.453592 kg. The U.S. pound is divided into 16 ounces.

There is more than one kind of pound in Britain. The Imperial Standard Pound is a mass equal to 0.45359237 kilograms. This is also the definition of the international pound. The United States agreed to this definition (although it didn’t adopt it) in 1959. Originally, one pound sterling was a tower pound of silver. In 1528, the standard changed so that one pound sterling became the Troy pound. The tower pound, London pound, and merchant’s pound are all obsolete units today. There is also the British Pound (GBP), which is a unit of currency rather than mass.


  • Fletcher, Leroy S.; Shoup, Terry E. (1978). Introduction to Engineering. Prentice-Hall. ISBN 978-0135018583.
  • United States National Bureau of Standards (1959). “Notices “Refinement of values for the yard and the pound“.
  • Zupko, Ronald Edward (1985). Dictionary of Weights and Measures for the British Isles: The Middle Ages to the 20th Century. DIANE Publishing. ISBN 0-87169-168-X.