The seven base units

Second (s)time

The second is fixed by the frequency of a particular transition in the caesium-133 atom: one second is exactly 9 192 631 770 of its cycles.

Δν_Cs = 9 192 631 770 Hz

This is the oldest of today’s definitions: the second has been tied to caesium since the 13th General Conference on Weights and Measures in 1967–1968. Atomic clocks built on this principle keep the world’s time; the BIPM combines their readings into Coordinated Universal Time (UTC).

In practice the second is realised by caesium fountain clocks in national metrology institutes. Optical clocks, which count the much faster oscillations of light, have become better still: some reach uncertainties about a hundred times smaller than the caesium fountains. That is why a new definition of the second is being prepared: in 2026 the General Conference asked for a proposal to be put to its next meeting, in 2030; caesium clocks will remain in use as secondary standards and nothing changes for everyday timekeeping.

Metre (m)length

The speed of light in vacuum is fixed exactly. A metre is the distance light covers in 1/299 792 458 of a second.

c = 299 792 458 m/s

Light has defined the metre since 1983. Before that, the metre began life in 1799 as a fraction of the Earth’s meridian, and from 1889 it was the length of a metal bar kept at the BIPM. The full story is on the History page.

In practice laboratories realise the metre with lasers whose frequency is known very precisely. The CIPM keeps a list of such standard frequencies, for example the line near 633 nm used to stabilise helium-neon lasers. Knowing the frequency and the fixed speed of light gives the wavelength, and wavelengths are what interferometers use to measure length.

Kilogram (kg)mass

The kilogram is defined by fixing the value of the Planck constant. Together with the second and the metre, that pins down the kilogram.

h = 6.626 070 15 × 10⁻³⁴ J s

From 1889 to 2019 the kilogram was the mass of a single object, the International Prototype of the Kilogram, kept at the BIPM. The value of h was chosen so that the kilogram did not change at the moment of the switch.

Two very different experiments realise the kilogram today. A Kibble balance weighs a mass against an electromagnetic force and compares electrical and mechanical power; the Planck constant links the two. The X-ray crystal density method counts atoms instead: the volume and crystal spacing of an almost perfect sphere of silicon-28 give the number of atoms in it, and from that its mass.

Laboratories that run such experiments compare their results in key comparisons organised by the BIPM, which also operates a Kibble balance. Until the realisations agree closely enough, the kilogram is passed on to the world from an agreed consensus value.

Ampere (A)electric current

The charge of a single proton (the elementary charge) is fixed exactly. An ampere is a flow of a fixed number of such charges per second.

e = 1.602 176 634 × 10⁻¹⁹ C

The old definition worked through the magnetic force between currents, which in effect fixed the magnetic constant μ₀. Since 2019 μ₀ is measured instead. For almost everyone nothing changed; at the very top of electrical metrology the volt shifted by about 0.1 parts per million.

Kelvin (K)temperature

The Boltzmann constant links temperature to energy. Fixing its value defines the kelvin.

k = 1.380 649 × 10⁻²³ J/K

Until 2019 the kelvin was tied to one property of one substance: the triple point of water was exactly 273.16 K. That temperature is now a measured value with a tiny uncertainty. The change made no difference to everyday thermometers, but it opens the way to better measurements at very low and very high temperatures.

Mole (mol)amount of substance

A mole is simply a count: exactly 6.022 140 76 × 10²³ elementary entities, such as atoms or molecules.

N_A = 6.022 140 76 × 10²³ mol⁻¹

The mole became a base unit in 1971. Until 2019 it was defined through mass, as the number of atoms in 0.012 kg of carbon-12, so it depended on the kilogram. Now it does not. Atomic weights and the chemist’s everyday practice stay as they were.

Candela (cd)luminous intensity

The candela is fixed by the luminous efficacy of green light at a frequency of 540 × 10¹² Hz, the colour the human eye is most sensitive to in daylight.

K_cd = 683 lm/W

This definition dates from 1979 and was carried over unchanged. The candela is the only base unit that measures something through the human eye, which is why its constant is a technical one rather than a constant of physics.