kg

The kilogram (symbol kg) is the SI base unit of mass. It is the reference unit for measuring mass in the International System of Units (SI) and is used across science, industry, and everyday life. A kilogram is equal to 1000 grams, and a gram is one thousandth of a kilogram. In common practice, the kilogram is also close to 2.20462 pounds (avoirdupois).

Definition and realization

Since 2019, the kilogram is defined by fixing the numerical value of the Planck constant h to exactly 6.62607015×10^-34 joule seconds. With the definitions of the metre and the second, this fixes the kilogram without relying on a physical artifact. Prior to 2019, the kilogram was defined as the mass of the International Prototype Kilogram (IPK), a platinum‑iridium cylinder stored at the International Bureau of Weights and Measures (BIPM) in Sèvres, France, with copies kept as replicas. The IPK defined mass for nearly a century and its copies served as reference standards.

Realization methods

Mass standards are realized through highly precise techniques that relate mass to fundamental constants. This includes instruments such as the Kibble (or watt) balance, which compares mechanical power to electromagnetic power, and atom counting experiments that connect mass to the Planck constant via the definition of the kilogram.

Usage

The kilogram remains the primary unit of mass in science and most of everyday commerce. Many derived units and prefixes (milligram, gram, tonne) are built from it, and it provides the basis for accurate measurement in research, manufacturing, and trade.