Pounds to Kilograms Converter — lb to kg

About Converting Pounds to Kilograms

Pound and Kilogram both show up in recipes, and converting between them matters whenever a recipe from one country's cookbook needs to be followed with another country's measuring tools. Pound is typically associated with united States (primary mass/weight unit), while Kilogram is typically associated with used as the standard mass unit in nearly every country except the United States and a few others that primarily use the pound..

Reverse conversion: Kilogram to Pound

For a page dedicated to this direction, see Kilogram to Pound.

Pound vs. Kilogram

Understanding the Pound

Pound (lb) measures mass. The avoirdupois pound is legally defined as exactly 0.45359237 kilograms, fixed by international treaty in 1959. It is classified as a US/imperial customary unit.

Where it's used: United States (primary mass/weight unit) United Kingdom (informal and some retail use) Various Caribbean and Pacific territories with historical UK/US ties

Pound-mass vs. pound-force. This page converts pound-mass (lbm), a unit of mass. Pound-force (lbf) is a separate unit measuring the force gravity exerts on one pound-mass, used in engineering contexts like pounds per square inch.

Uses of the Pound today

• Body weight and food retail packaging in the United States
• Postal, parcel, and freight weight in US shipping and logistics
• Pound-force (lbf) and its derivatives (psi, foot-pounds) in US engineering and aerospace

History of the Pound

Unlike the kilogram's single documented birth in revolutionary France, the pound's history is a long, layered narrowing of competing regional standards. The avoirdupois pound — ancestor of today's standard — emerged among London merchants in the early 14th century (recorded use around 1303), used for weighing wool and other bulk goods under the system merchants called 'avoir de pois,' Old French for 'goods of weight.' Its 16-ounce structure suited large-scale trade better than the 12-ounce troy pound used for precious metals.

Wool merchants used a related 'wool pound' of about 6,992 grains — close enough to 7,000 that Elizabeth I's government formally fixed the avoirdupois pound at exactly 7,000 grains in 1588, the value still used today. Britain and the US then refined their physical standards separately over the following centuries, leaving small discrepancies between them.

Those discrepancies were resolved on 1 July 1959, when the US, UK, Canada, Australia, New Zealand, and South Africa signed the International Yard and Pound Agreement, fixing the pound at exactly 0.45359237 kg and the yard at exactly 0.9144 m. The US National Bureau of Standards (now NIST) adopted the value immediately; the UK followed via its Weights and Measures Act 1963 — the value used by virtually every country today.

Historical origin. Pound was not the invention of a single named person; it developed through the process described above.

Understanding the Kilogram

Kilogram (kg) measures mass. The kilogram is the SI base unit of mass; every other SI mass unit is a decimal multiple or fraction of it. It is classified as a SI base unit.

Where it's used: Used as the standard mass unit in nearly every country except the United States and a few others that primarily use the pound.

Uses of the Kilogram today

• Pricing and labeling goods sold by weight in retail and commodity trade
• Dosing calculations in medicine, especially for pediatric and weight-based prescriptions
• Structural and material specification in metric-system engineering
• Scientific measurement of mass across chemistry, physics, and materials research

History of the Kilogram

The kilogram's direct predecessor was the 'grave,' defined by the French Academy of Sciences in 1793 as the mass of one cubic decimeter of water at its temperature of maximum density. Because 'grave' echoed the aristocratic title 'graf,' revolutionary France's decree of 18 Germinal, Year III (7 April 1795) renamed the system: the gram became the reference unit, defined as the mass of one cubic centimeter of water at the melting point of ice, and the kilogram — one thousand grams — took over the grave's practical role.

Turning that water-based definition into a physical object required real experimental work: chemist Louis Lefèvre-Gineau was tasked in 1795 with measuring precisely how much a cubic decimeter of water weighed, and former royal jeweller Marc-Étienne Janety then fabricated a platinum cylinder to embody that measured mass. Deposited in the French National Archives in 1799, it became known as the Kilogramme des Archives.

The metric system spread unevenly — Napoleon briefly restricted its use — but France made it compulsory in 1840, and the 1875 Metre Convention (signed by seventeen nations including the US) established the BIPM and a shared international prototype. The platinum-iridium International Prototype of the Kilogram, adopted in 1889 and nicknamed 'Le Grand K,' served as the world's mass reference for over a century.

On 20 May 2019, the kilogram was redefined by fixing the Planck constant at exactly 6.62607015 × 10⁻³⁴ joule-seconds, so it can now be realized in a laboratory (via a Kibble balance) without reference to any physical object.

Historical origin. Kilogram was not the invention of a single named person; it developed through the process described above.