Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

The SI Redefinition: Background Information

Scientists Voted on Metric Makeover - English
Scientists Voted on Metric Makeover - English

In 2019, a metal cylinder in a vault outside Paris will be replaced with a new way to measure mass based entirely on the laws of nature. 

Why is this important? 
The metal cylinder, known as the International Prototype Kilogram (aka “Big K”) is the kilogram. It is the standard for measuring mass in almost every nation on Earth. It is also, however, an antiquated artifact of a bygone era.

Big K is indeed the last “artifact,” or physical prototype of measurement, of the SI. It represents not only the mass of the kilogram, but an idea that goes back thousands of years, when actual physical objects defined all measurement systems. Think ‘hands,’ ‘stones,’ ‘feet,’ and ‘cubits.’ 

Over the course of the twentieth century, some of the metric units of measurement were redefined in terms of natural constants. Measurement standards defined by natural phenomena are exceptionally accurate, precise and reproducible. But the kilogram, and the measurement units derived from it (such as force, which is measured in newtons), were not among them, until a vote on November 16, 2018, changed the way the world measures mass. 

There is a reason the kilogram had not been redefined until recently, and why “Big K” still had a job: devising something better has been extremely difficult. The new definition starts with an agreed value for a quantum-mechanical quantity called Planck constant (represented by a lowercase h).

NIST in 90: Measuring Planck’s Constant
NIST in 90: Measuring Planck’s Constant

In 1999, NIST scientists first proposed the idea of defining the kilogram by assigning a fixed value to the Planck constant. This would be done by using a Kibble balance, a complex device named after scientist Bryan Kibble, who conceptualized it in 1975. The Kibble balance matches mechanical power, such as that generated by a moving kilogram mass, to an equivalent amount of electrical power, such as that generated by the voltage in a special coil in the device. The amount of voltage in the coil is proportional to Planck’s constant h. So, the Kibble balance could first use an exactly defined mass to measure the value of h, and then reverse the process: by using an exact fixed value of h, the same system could measure an unknown mass. A second proposed method would define the kilogram in terms of the mass of a silicon atom by counting the number of atoms in a 1 kg sphere of ultra-pure silicon-28 (the most abundant isotope of silicon, which contains a total of 28 protons and neutrons). 

The General Conference on Weights and Measures, which oversees the International Bureau of Weights and Measures, endorsed the idea of using these new methods to redefine the kilogram with a few requirements, including: 1) at least three independent experiments had to produce a mass measurement that was accurate to 50 parts per billion, 2) at least one of them had to be accurate to within 20 parts per billion, AND 3) the results of the Kibble balance and silicon sphere methods had to agree with each other to a statistical confidence level of 95 percent. The agreed-upon value of Planck’s constant was established in October 2017, and a paper about this new value was published in January 2018.

The improvements to measurement science made possible by the redefinition of the kilogram will allow unprecedented scalability, meaning that measurements of the impossibly large to the vanishingly small will be linked through fundamental constants of nature.

If experience is an indicator, the implications for the future will be compelling. The 20th century redefinitions of the second and the meter enabled the development of GPS, the internet, and interstellar navigation. The vote to redefine the kilogram redefined not only how to measure a kilogram, but also three other measurement units, those used for measuring temperature (kelvin), electricity (ampere), and amounts of substance (mole).

The decades of work put into the kilogram redefinition culminated in a November 16, 2018, meeting of the General Conference on Weights and Measures (Conférence Générale des Poids et Mesures, CGPM) at the Palace of Versailles in France. Attended by scientists from around the world, the representatives from more than 50 nations came together and voted to officially implement the new definition of the kilogram and the units derived from it. The world’s nations will begin to implement these new standards in May 2019. Therefore, U.S. schools will need either new or updated curricula to teach students about this major improvement in how measurements in industry and science will be made.


Created November 27, 2018, Updated November 28, 2018