Kilogram (kg)

Definition

The SI base unit of mass, fixed since 2019 by defining the Planck constant as exactly 6.62607015×10⁻³⁴ J·s.

History

The kilogram began in 1795 as the mass of one liter of water at the melting point of ice, and from 1889 was embodied in a platinum-iridium cylinder kept near Paris, the International Prototype Kilogram. In 2019 the SI retired that physical artefact and redefined the unit through a fixed value of the Planck constant, leaving its size unchanged.

Where it is used

It is the base unit of mass almost everywhere on Earth. The main everyday exception is the United States, where people weigh in pounds even though US science, medicine and industry work in kilograms.

Typical uses include body weight outside the US, groceries and produce, checked-luggage limits.

Conversion pages

These pages convert kilogram to or from another unit in weight.

• Kilogram to pound
• Kilogram to stone
• Ounce to kilogram
• Kilogram to tonne

Other Weight units

• Gram (g)
• Pound (lb)
• Ounce (oz)
• Stone (st)
• Milligram (mg)
• Microgram (mcg)
• Tonne (t)

Sources

• BIPM SI BIPM, The International System of Units (SI), 9th ed. (2019) · Dataset reviewed Nov 11, 2026