How the kilogram is realized

The SI unit of mass

How do we know that one kilogram is the same everywhere in the world?

The kilogram is the SI unit of mass. For more than a century, it was defined by a single platinum-iridium artefact conserved at the BIPM. This ensured a common reference for the world, but also meant that the definition of the kilogram depended on one physical object.

A major milestone was reached in 2019, when the International System of Units (SI) was revised and the kilogram became defined by fixing the numerical value of the Planck constant, a fundamental constant of nature. As a result, the kilogram can now be realized consistently around the world without relying on a single artefact.

The BIPM supports the international realization and dissemination of the kilogram through the operation of its Kibble balance, mass calibration services, international comparisons and coordination with National Metrology Institutes (NMIs).

For measurement specialists

Explore the technical detail

This page provides a simple overview. For more detailed technical information, use the BIPM resources below.

How is the kilogram defined today?

The kilogram is the SI unit of mass. Since 20 May 2019, it has been defined by fixing the numerical value of the Planck constant, a fundamental constant of nature. This allows the kilogram to be realized without relying on a single physical artefact.

What is a Kibble balance?

A Kibble balance is an instrument used to realize the kilogram by linking mechanical measurements of mass to electrical measurements connected to the Planck constant.

Does the International Prototype of the Kilogram still define the kilogram?

No. The International Prototype of the Kilogram (IPK) defined the kilogram until the revised SI came into force on 20 May 2019. The BIPM continues to conserve the IPK as part of the history of the SI and the evolution of international mass metrology.

Are Kibble balances the only way to realize the kilogram?

No. The kilogram can also be realized using the X-ray Crystal Density (XRCD) method, which determines mass through measurements of the number of atoms in a highly characterized silicon crystal. The mise en pratique for the kilogram describes the recognized methods for realizing the unit.

How does the BIPM support the international realization of the kilogram?

The BIPM supports the international realization and dissemination of the kilogram through the operation of its Kibble balance, mass calibration services, international comparisons and coordination with National Metrology Institutes (NMIs).

Why are international mass comparisons important?

The results of key comparisons support the international recognition of Calibration and Measurement Capabilities (CMCs) under the CIPM MRA. Approved CMCs are published in the BIPM Key Comparison Database (KCDB).