Change in the definition of basic units of the SI system

Today we can talk literally about the revolution in metrology. Up to four of the seven basic units of the SI system – kilogram, ampere, kelvin and mol – are redefined in a fundamentally different way than before.

What is this change?

This change consists in completing the transformation of the definitions of units based on the materialized form of the etalons into new definitions of units based on the physical foundations themselves, represented by the basic physical constants. In the newly defined SI system of units valid from 20 May 2019, seven physical constants are fixed, whose values are declared as exact numbers, which will no longer be changed and refined (they have zero uncertainty). The definitions of the seven basic units of the SI system are then derived from these seven physical constants.

These are the constants:

  • ΔνCs      frequency of hyperfine transition of the Cesium atom 133Cs: 9 192 631 770 Hz
  • c             speed of light in vacuum: 299 792 458 m.s−1
  • h             Planck constant: 6,62607015 × 10−34 J.s (kg.m2.s−1)
  • e             elemental charge: 1,602176634 . 10−19 C
  • k             Boltzmann constant: 1,380649 .10−23 J. K−1
  • NA          Avogadro constant: 6,02214076 . 1023 mol−1
  • Kcd    light efficiency of monochromatic radiation of frequency 540.1012 Hz: 683 lm. W−1

What are the reasons behind these changes?

The reason for these changes is the intention to completely exclude from the definitions of SI basic units physical standards that may be subject to change in the long run, and they are also state-of-the-art requirements to increase the accuracy and scope of measurements, e.g. in nanotechnologies, microelectronics, precision engineering, the aerospace industry, chemistry and many other areas of human activity.

Let’s now look at the individual units, whose definitions change from 20. May 2019 .


The kilogram is the basic SI unit of weight. So far, the kilogram has been defined as the weight of the international prototype kilogram. This standard is stored at the International Bureau of Weights and Measures in Sèvres, France, and is made as a 9: 1 platinum-iridium alloy cylinder. This definition of the kilogram is easy to understand even for people without physical education, it has survived for more than a hundred years, but due to potential fluctuations in the weight of the prototype kilogram, it no longer meets the needs of modern metrology and industry.

The new definition of a kilogram is as follows [1]:

  • Kilogram, the symbol kg, is the SI unit of weight. It is defined such that the numerical value of the Planck constant h is exactly 6,62607015.10−34 when expressed in the unit J.s, which is equal to kg.m2.s−1, where meter and second are defined by c and ΔνCs.

When we first read this definition, probably all of us who remember the old definition of a kilogram from high school will be taken aback. What does a kilogram have to do with Planck’s constant, introduced in quantum physics?

The new definition of the kilogram from 2019 is therefore principally based on the declaratory definition of the value of the physical constant – Planck’s constant h = 6,62607015.10− The value of this constant has been refined for a long time by physical measurements, and now we will surprisingly assign it a fixed value !? It’s hard to understand, so let’s try to explain.

The physical dimension of the Planck constant is J.s (Joule second), or kg.m2.s−1. However, it follows that if we precisely define the value of the Planck constant, the weight of the kilogram can be derived from two other basic SI units acting in the physical dimension of this constant, namely the unit of length – meter m and the unit of time – second s. However, it is necessary to find a suitable experiment, which will be described in the section How to implement a kilogram in accordance with the new definition.

What are the consequences? Such that the definition of the SI kilogram unit ceases to be dependent on the materialized form of the kilogram standard, the mass value of which may change over time (eg by sorption of chemical impurities from the atmosphere on the surface of the standard). The new definition of the kilogram is based only on the exact value of the Planck constant and the other two basic units SI – meter and second. The definitions of these two units are also related to the exact physical constants, the second to the frequency of the hyperfine transition of the ground state of the Cesium atom Cs and the meter to the speed of light in vacuum.

How to implement a kilogram in accordance with the new definition

At present, there are in principle two sufficiently precise methods applicable to the implementation of the kilogram unit in accordance with the new definition. One method involves counting atoms in a pure silicon sphere 28Si, which weighs the same value as the reference kilogram. This can be used to calculate the value of the Avogadro’s constant, which can be further transformed to the value of the Planck’s constant. The second method uses an instrument known as the Kibble weight to obtain the value of the Planck constant by comparing the gravitational force on the test mass with the electromagnetic force on a coil flowing in a magnetic field (using the Josephson and Hall quantum effect). In recent years, the results of both methods have been in perfect agreement and are sufficiently accurate to meet the requirements for a new definition of the kilogram.

After understanding this new philosophy used in defining the basic unit SI kilogram, it should no longer be a problem to understand the new definitions of the other three redefined units: ampere, kelvin and mol.


  • Ampere, symbol A, is the SI unit of electric current. It is defined so that the numerical value of the elementary charge e is exactly 1.602176634.10−19, when expressed in unit C, equal to A.s, where the second is defined by ΔνCs.


  • Kelvin, the symbol K, is the SI unit of thermodynamic temperature. It is defined such that the numerical value of the Boltzmann constant k is exactly 1.380649.10−23 when expressed in the unit J.K−1, which is equal to kg.m2.s–2.K–1, where kilogram, meter and second are defined by h, c and ΔνCs.


  • The mol, the symbol mol, is the SI unit of substance. One mole contains exactly 6.02214076.1023 elementary entities. This number is a fixed numeric value of the Avogadro’s constant NA when expressed in mol-1 and is called the Avogadro’s number. The quantity of a substance, the symbol n, of the system is a measure of the number of specified elementary entities. These entities can be atoms, molecules, ions, electrons, various other particles, or specified groups of particles.

What will change the definition of SI units mean for humanity?

Current knowledge in physics assumes that the laws of physics apply in their known form equally everywhere on Earth, as well as the places available to mankind in the Universe. Therefore, even the fundamental physical constants used in the new definitions of SI units have the same values there and can be used to implement basic SI units. One of the hidden consequences of the new definition of SI units is the possibility of implementing primary standards of SI units everywhere on Earth, and in the near Universe with the same uncertainty, which will allow further development and expansion of modern technologies on Earth and in the Universe. However, it should be emphasized that the redefinition of SI units will not be adversely affected by the average person and e.g. the new kilogram will be the same size as the old one …


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