Kilogram: SI base unit of mass

Definition and modern redefinition

Since 20 May 2019 the kilogram is defined by fixing the numerical value of the Planck constant to exactly 6.62607015×10⁻³⁴ joule seconds (J·s). Because the metre and the second are already defined in terms of physical constants, fixing the Planck constant provides an exact, invariant basis for the kilogram. This replaced the earlier artifact-based definition that relied on the International Prototype Kilogram (IPK), a platinum–iridium cylinder stored near Paris.

How the kilogram is realized in practice

Modern laboratories implement the kilogram definition using precision methods that connect mass to the fixed Planck constant. Two widely used approaches are:

• Kibble balance (formerly called a watt balance): an electromechanical instrument that compares mechanical power to electrical power referenced to quantum electrical standards. The balance relates mass to electrical measurements traceable to the fixed Planck constant.
• Avogadro or silicon-sphere method: highly pure, nearly perfect spheres of silicon-28 are measured to determine the number of atoms and the lattice spacing. That atom-counting approach links macroscopic mass to a defined quantity of atoms (atoms).

Historical background

Before the 2019 redefinition, the kilogram was represented by the International Prototype Kilogram (IPK), created in the late 19th century. Over time small mass differences appeared between the IPK and national copies, motivating a shift to a definition based on unchanging constants of nature. Earlier still, metric ideas in the 18th and 19th centuries tied the gram to the mass of a volume of water (the litre), with water's maximum density near 4 °C playing a historical role in defining mass units (litre, sea level conditions).

Common relations and conversions

Practical comparisons and everyday conversions include: one kilogram is slightly more than two pounds—about 2.20462 pounds. One kilogram subjected to standard gravity (9.80665 m/s²) corresponds to a weight of about 9.80665 newtons. The metric hierarchy uses the gram as 1/1000 of a kilogram and the tonne (or metric ton) equals 1,000 kilograms.

Distinctions and notable facts

It is important to distinguish mass from weight: mass (measured in kilograms) is an intrinsic property of matter; weight depends on local gravity. The 2019 redefinition makes the kilogram stable and the same anywhere in the universe because it is tied to fundamental constants rather than a physical object. The fixed value of the Planck constant now anchors mass measurements to the same immutable framework that defines the metre and the second.

Key practical points include:

1. The kilogram is the SI base unit of mass.
2. It is realized experimentally by instruments such as the Kibble balance and atom-counting techniques using silicon spheres.
3. Historically linked to the mass of a litre of water at a temperature near 4 °C, a relation that helped establish the gram in the 18th century.

For further technical information and national metrology resources see metrology organizations and review articles (unit definition, SI, scientific context).