Kilogram

The kilogram is the base unit of mass in the International System of Units (SI) and is defined as exactly one thousand grams. Represented by the symbol kg, it is the only SI base unit whose official name incorporates a metric prefix. The term derives from a combination of kilo, meaning one thousand, and gram, and is widely used both in scientific contexts and everyday measurement. Although the informal term kilo is common in colloquial usage, scientific and technical standards require the full unit name or symbol.

Historical Development of the Kilogram

The origins of the kilogram date to the late eighteenth century during the period of metric reform initiated by the French Revolution. In 1793 the earliest precursor, the grave, was defined as the mass of one litre of water. This was refined in 1795 when the gram—one-thousandth of a kilogram—was provisionally defined as the mass of one cubic centimetre of water at the melting point of ice. These early definitions reflected the revolutionary ambition to base units on natural physical properties rather than arbitrary artefacts.
In 1799 the Kilogramme des Archives, a platinum prototype, was produced to serve as the physical representation of the unit. It was based on the mass of one cubic decimetre of water at its maximum density, approximately 4°C. This artefact remained the reference until international cooperation on standardisation led to the signing of the Metre Convention in 1875. The Convention provided the framework for creating a uniform system of measurements, resulting in the manufacture of the platinum–iridium International Prototype of the Kilogram (IPK) in 1879 and its adoption in 1889.
For more than 130 years the IPK served as the global standard for mass. However, long-term comparisons revealed that the prototype and its official copies were slowly diverging, with differences of around 50 micrograms accumulating over time. This gradual drift raised concerns about relying on a single physical object to define a fundamental unit.

Definition Based on Physical Constants

On 16 November 2018 the General Conference on Weights and Measures (CGPM) approved a sweeping revision of the SI, redefining the kilogram in terms of invariant physical constants. From 20 May 2019 the kilogram has been defined by fixing the value of the Planck constant (h) at an exact number when expressed in joule-seconds.
This definition makes use of three constants underpinning the SI framework:

• A specific atomic transition frequency of caesium, which defines the second.
• The speed of light in vacuum, which defines the metre when combined with the second.
• The Planck constant, which defines the kilogram when combined with the metre and the second.

Through this formulation, mass measurement becomes anchored to fundamental properties of nature rather than to a physical artefact. Laboratories equipped with a Kibble balance or other advanced metrological instruments can independently realise the kilogram by relating mechanical power to electromagnetic power through the defined value of the Planck constant.
The new standard aligns closely with previous definitions, remaining within approximately 30 parts per million of the mass originally intended by the water-based definition of the late eighteenth century.

Timeline of Key Definitions

The development of the kilogram reflects an ongoing effort to improve accuracy, reproducibility, and scientific coherence:

• 1793: The grave defined as the mass of one litre of water.
• 1795: The gram provisionally defined as the mass of one cubic centimetre of water at the melting point of ice.
• 1799: Manufacture of the Kilogramme des Archives, based on the mass of one cubic decimetre of water at maximum density.
• 1875: Signing of the Metre Convention, initiating the creation of international prototypes.
• 1889: Formal adoption of the platinum–iridium International Prototype of the Kilogram as the world standard.
• 2019: Redefinition of the kilogram in terms of the Planck constant, the speed of light, and the caesium atomic clock transition frequency.

These stages mark the transition from material prototypes to definitions rooted in immutable principles of physics.

Mass Measurement and the Kibble Balance

Following the 2019 redefinition, the most prominent method of realising the kilogram is the Kibble balance, an instrument capable of comparing mechanical and electrical power with extraordinary precision. The balance equates the gravitational force on a test mass with an electromagnetic force generated through a coil and magnet system. By measuring electrical quantities based on the quantum Hall and Josephson effects—both of which depend on fixed fundamental constants—metrologists can produce primary mass standards without reference to a physical object.
This technique ensures long-term stability of the unit and allows mass standards to be reproduced consistently across laboratories worldwide. Secondary standards, such as metal calibration weights, can now be derived from these primary realisations rather than handed down from a central artefact.

Name, Etymology, and Usage

The term kilogramme was written into French law in 1795 and subsequently adopted into English later that year. In British usage both spellings—kilogram and kilogramme—are accepted, although modern practice overwhelmingly favours kilogram. In the United States kilogram is standardised. British and international legislation regulating units of measurement does not restrict the choice of spelling.
The word’s linguistic roots lie in French, Greek, and Latin. The prefix kilo comes from the Greek khilioi, meaning a thousand, while gramme derives from a classical term referring to a small weight. Over time the clipped form kilo entered English, although scientific guidelines discourage its use in formal writing.
The SI Brochure specifies that unit symbols must not be abbreviated further; thus kg is the only correct symbol for the kilogram. For certain East Asian character sets, a dedicated Unicode symbol exists for compatibility purposes.

Redefinition Motivations and Scientific Significance

Replacing the International Prototype of the Kilogram required significant advances in measurement technology. Several decades of research showed that the IPK’s mass was not perfectly stable, diverging from its replicas despite careful storage and minimal handling. This divergence provided strong motivation to develop a definition based on physical constants, ensuring permanence and universality.
The 2019 redefinition also brought consistency across SI units. The metre had already been redefined in 1960—first using a spectral line of krypton, later the speed of light—allowing laboratories to reproduce it independently. Similarly, redefining the kilogram allowed comparable independence for mass measurements, aligning with the broader SI objective of basing all units on fixed constants of nature.
The change further influenced derived units. For example, the ampere no longer depends on the kilogram for its definition, instead relying on elemental charges and quantum electrical standards. This coherence strengthens the scientific foundation of the SI, facilitating more precise research in fields such as particle physics, cosmology, materials science, and precision engineering.

Contemporary Applications and Relevance

Today the kilogram is central to trade, industry, science, and regulation. Manufacturing processes rely on accurately calibrated masses; pharmaceuticals require stringent control over quantities; and scientific experiments depend on consistency of mass scales across international laboratories. The transition to a constant-based definition ensures that such applications can proceed with confidence even as technology continues to evolve.
Metrology institutes worldwide now calibrate mass standards by realising the kilogram through Kibble balances or silicon sphere methods, enabling high-precision consistency across national boundaries. The redefinition thus supports both scientific progress and practical measurement, ensuring that the kilogram remains a stable and reliable foundation of the SI measurement system well into the future.