Understanding the Redefining Moment of SI Base Units

SI units blog

The world of science and technology is constantly evolving, in more ways than we can imagine. A lot of things we studied in our school days as hard facts are now undergoing changes and evolution. One such change is being made in our measurement units. Weird, right! How can measurements be indefinite? How can a kilogram not have a universally-defined base value? Does that mean you’ll have to re-learn all these base measurements again?

It is to clear this uncertainty that the 26th General Conference of Weights and Measures voted unanimously in favour of revising definitions of the SI base units. This was a historic decision, taken on 16th November 2018, with delegates from 60 countries voting to bring a drastic change in the International System of Units. This system forms the foundation of global science and trade but had been a cause for concern for scientists around the world. The new definitions will be implemented from 20th May 2019.


Informally known as the metric system, the International System of Units is integral to measuring everything – from grains to galaxies. It has seven base units from which all other measurement units can be derived. Four of these seven units – Kilogram, Kelvin, Ampere, and Mole are slated to be redefined in terms of constants of nature. Let’s take a closer look to understand this change.

How was Kilogram Measured Earlier?

Since the 19th century, Kilogram’s definition has been based on the mass of Le Grand K or Big K, a 139-year old platinum-iridium cylinder kept inside a triple-locked vault at the International Bureau of Weights and Measures, France. However, the Big K is vulnerable. Its mass changes with the passage of time (even if a tiny amount) and could create chaos if it were to be stolen or destroyed. Even a small change in this cylinder could trigger changes in the entire global system of measurement. It was crucial to ensure that we stopped defining kilogram based on a physical thing.

The New Proposal

With the unprecedented advancement in science, more accurate measurements are achievable now. The revised measurement system stresses on defining all measurement units on a foundation of unchanging and eternal ‘universal constants’. The Kilogram will now be defined in terms of Planck’s constant, an unchanging numerical figure which describes the smallest possible unit of energy. Affixing Kilogram’s definition to Planck’s constant posed a big challenge and so came the new king – the Kibble balance – an exquisite complex machine used to precisely calculate the value of the Planck’s constant so that it can be used to accurately define a Kilogram. The Kilogram is now based on the fixed value of the Planck constant h, calculated to be 6.62607015 × 10−34 m2 kg / s.

With the Kilogram being redefined in terms of Planck’s constant, the Kelvin is being redefined in terms of the Boltzmann constant, the Ampere in terms of the elementary electric charge and the Mole in terms of the Avogadro’s number.

However, do not let the new definitions worry you. The revised definitions aim to improve the SI Units without causing a change in any of the size of these units, thus ensuring continuity with existing measurements. Remember, we at Extramarks are always there to lead you through and keep you up-to-date on any new developments in the field of science, technology, and education.