SI units > Redefining the SI units

# Redefining the SI units

As science advances, ever more accurate measurements are both required and achievable. The standard and definition for each unit must reflect this increasing accuracy.

The kilogram is based on a physical object certified in 1889, and it is the last unit to be based on an actual object. Its stability has been a matter of significant concern, resulting in recent proposals to change the definition to one based on natural constants.

From May 2019, all the base units are expected to be defined in terms of natural constants, which are the most stable things we have ever encountered. It is anticipated that a revision to the SI units will be agreed in November 2018. As with previous re-definitions of units, we cannot foresee all the consequences, but we expect them to form foundations for decades of improved measurements.

## Redefinition

The new definitions impact four of the base units: the kilogram, ampere, kelvin and mole:

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

## Expected 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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