# SI base units

Measurement of any quantity involves applying a value to it by comparing it with some precisely defined unit value of the quantity. These standard units form the basis of a globally recognised language, which allows measurements to mean the same thing no matter where they’re made.

### The seven SI base units

The International System of Units is the most widely used system of measurement. The SI base units were revised in 2019 so each is defined by a constant of nature.

Image reproduced with permission of the BIPM, which retains full internationally protected copyright.

**Rights:**Image reproduced with permission of the BIPM, which retains full internationally protected copyright.

At the heart of this is metrology, which literally translates as the science of measurement It comes from two Greek words: ‘metron’ = measure and ‘logos’ = the study of.

Metrology establishes a common way of expressing measurements at all levels of precision, based on the International System of Units (the SI). The SI is made up of seven universally recognised units, but other units – like velocity (m/s) – are derived from these seven. Other non-SI units, such as hours or tonnes, can also be related back to the SI.

The SI was formally revised on 20 May 2019. Changes had been made to the SI a number of times throughout its history, but this latest update was unprecedented and required simultaneous worldwide agreement across diverse fields of metrology. Four of the base units – kilogram, ampere, kelvin and mole – were redefined based on fixed values for some natural constants, including Planck’s constant.

Quantity measured (and te reo Māori translation)

Unit

Symbol

Definition

time (wā)

second (hākona)

s

The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the 133Cs atom.

length (roa)

metre (mita)

m

The metre is the distance travelled by light in a vacuum in exactly 1/299,792,458 s.

mass (papatipu)

kilogram (manokaramu)

kg

The kilogram is defined by fixing the value of h = 6.626 070 15 × 10–34 kg m2 s−1 and combining it with the definitions of the metre and the second, which rely on the speed of light and a specified transition frequency of a caesium atom, respectively.

thermodynamic temperature (pāmahana wera ahupūngao)

kelvin (kelvin)

K

The kelvin is equal to the change of thermodynamic temperature that results in a change of thermal energy k T by 1.380 649 x 10–23 J.

amount of substance (rahinga matū)

mole (tīwhanga)

mol

The mole is the amount of substance of a system that contains 6.022 140 76 x 1023 specified elementary entities.

electric current (iahiko)

ampere (wae-iahiko)

A

The ampere is the electric current corresponding to the flow of 1/(1.602 176 634 x 10–19) elementary charges per second.

luminous intensity (kukū whakaputa tūrama)

candela (kānara)

cd

The candela is the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 W/sr (watts per steradian).

## SI base units explained

The interactive map below has text and video that provides information about each of the SI base units.

## Nature of science

The aim of science is to explain phenomena by using empirical evidence. Data collection often involves measurement, and standard units of measure need to be identified and defined as accurately as possible to enable consistency and comparison.

## Related content

These articles explain other units of measurement:

These resources explain how to write and/or express the unit symbols:

## Useful links

The Measurement Standards Laboratory of New Zealand has prepared PDF translation of the BIPM’s Concise Summary of the SI in te reo Māori that can be downloaded here.

BIPM is the intergovernmental organisation through which Member States act together on matters related to measurement science and measurement standards.

This informative and fun animation How big is a mole? (Not the animal, the other one.) teaches students about the concept of the mole in chemistry.

## Acknowledgement

This resource has been updated with the assistance of the Measurement Standards Laboratory of New Zealand.

### MSL logo

The Measurement Standards Laboratory of New Zealand (MSL) is New Zealand’s national metrology institute. It ensures that New Zealand’s units of measurement are consistent with the SI, the international system of units.

**Rights:**Measurement Standards Laboratory

**Published:**17 August 2011