SI units > Redefining the SI units

# Redefining the SI units

As science advances, ever more accurate measurements are both needed and achievable. But this improving accuracy must happen through measurement standards and their definitions.

In November 2018, at the General Conference on Weights and Measures, the global metrology community agreed a revision to the SI.

The decision means that, for the first time, all seven of the base units will be defined in terms of constants of nature – such as the speed of light, the Planck constant and the Avogadro constant. Using seven defining constants as the basis for the SI will mean that the definitions of all the base units will stay stable into the future.

The revision will bring in new definitions of the ampere, kilogram, kelvin (and, consequently, degree Celsius) and mole.

Although these changes won't be felt in everyday life, they represent a profound change of perspective. From May 2019, all the base units of the SI will be defined in terms of constants of nature – the most stable quantities we have ever encountered.

## Redefinition

Making this revision across the whole SI is a profound change in approach that will underlie all measurements in science and more widely. These changes will be used by scientists and engineers who are making measurements at the extremes, but in everyday life it will appear that not much has changed. The changes in the SI will ensure that the SI definitions stay robust for the future, ready for advancements in science and technology. The new definitions impact four of the base units:

- The kilogram – will be defined in terms of the Planck constant (
*h*) - The ampere – will be defined in terms of the elementary charge (
*e*) - The kelvin – will be defined in terms of the Boltzmann constant (
*k*) - The mole – will be defined in terms of the Avogadro constant (
*N*)_{A}

## Definitions from May 2019

### kilogram (kg)

**The kilogram is the SI unit of mass**

The kilogram is defined by taking the fixed numerical value of the Planck constant *h* to be 6.626 070 15 × 10^{-34} when expressed in the unit J s, which is equal to kg m^{2} s^{−1}, where the metre and the second are defined in terms of *c* and *∆ν*.

### metre (m)

**The metre is the SI unit of length**

The metre is defined by taking the fixed numerical value of the speed of light in vacuum *c* to be 299 792 458 when expressed in the unit m s^{−1}, where the second is defined in terms of the caesium frequency *∆ν*.

### second (s)

**The second is the SI unit of time**

The second is defined by taking the fixed numerical value of the caesium frequency *∆ν*, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s^{−1}.

### ampere (A)

**The ampere is the SI unit of electric current**

The ampere is defined by taking the fixed numerical value of the elementary charge *e* to be 1.602 176 634 × 10^{−19} when expressed in coulombs, which is equal to A s, where the second is defined in terms of *∆ν*.

### kelvin (K)

**The kelvin is the SI unit of thermodynamic temperature**

The kelvin is defined by taking the fixed numerical value of the Boltzmann constant *k* to be 1.380 649 × 10^{−23} when expressed in the unit J K^{−1}, which is equal to kg m2s^{−2} K^{−1}, where the kilogram, metre and second are defined in terms of *h*, *c* and *∆ν*.

### mole (mol)

**The mole is the SI unit of amount of substance**

One mole contains exactly 6.022 140 76 × 10^{23} elementary entities. This number is the fixed numerical value of the Avogadro constant, *N _{A}*, when expressed in the unit mol

^{–1}and is called the Avogadro number.

### candela (cd)

**The candela is the SI unit of luminous intensity in a given direction**

The candela is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 10^{12} Hz, *K*_{cd}, to be 683 when expressed in the unit lm W^{−1}, which is equal to cd sr W^{−1}.

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