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SI units

The SI base units

The globally-agreed system of measurement units was formally named the 'International System of Units' (SI) in 1960. The SI covers units for every type of measurement, but at the heart of the SI is a set of seven units known as the ‘base units’.

kilogram (kg) Unit of mass
metre (m) Unit of length
second (s) Unit of time
ampere (A) Unit of electric current
kelvin (K) Unit of thermodynamic temperature
mole (mol) Unit of amount of substance
candela (cd) Unit of luminous intensity

This International System of Units is necessary to ensure that our everyday measurements remain comparable and consistent worldwide. Standardising such measurements not only helps to keep them consistent and accurate, but also helps society have confidence in data. For instance, mass is measured every day, and having agreement on the definition of the kilogram means that consumers can trust that the shop is really providing the mass they say they are. Equally, having reliable information on climate change, pollution and medical diagnostics is important to society and builds trust, allowing effective decisions to be made.

Find out more about the redefinition of the SI units

How are the units of measurement defined?

Historically, units of measurement were defined by physical objects or properties of materials. For example, the metre was defined by the length between lines engraved on a metal bar and the kilogram was defined as the mass of a single cylinder of platinum-iridium metal – the International Prototype of the Kilogram (IPK).

In these two examples, the definition was also the realisation – the physical form – of the unit. However, such physical representations can change over time and are susceptible to damage or loss. So, over the years, the definitions have evolved to depend instead on constants of nature that are more stable and reproducible, meeting the demanding needs of today’s research and technological applications.

During the last century, scientists measured constants of nature, such as the speed of light and the Planck constant, with increasing accuracy. They discovered that these were far more stable than physical objects. It became clear that these constants of nature could offer a new and more stable foundation for the SI.

Find out about our current research on SI units

We welcome the opportunity to deliver technical lectures on metrology and SI units at universities and other organisations, please contact us to discuss your requirements.

SI prefixes

SI prefixes are used to form decimal multiples and submultiples of SI units. They should be used to ensure numerical values presented remain on the ‘human scale’ – ideally between 1 and 100. The grouping formed by a prefix symbol attached to a unit symbol constitutes a new inseparable unit symbol.

Multiplying Factor Name (symbol) Scientific Notation
1 000 000 000 000 000 000 000 000 yotta (Y) 1024
1 000 000 000 000 000 000 000 zetta (Z) 1021
1 000 000 000 000 000 000 exa (E) 1018
1 000 000 000 000 000 peta (P) 1015
1 000 000 000 000 tera (T) 1012
1 000 000 000 giga (G) 109
1 000 000 mega (M) 106
1 000 kilo (k) 103
100 hecto (h) 102
10 deca (da) 101
1   100
0.1 deci (d) 10-1
0.01 centi (c) 10-2
0.001 milli (m) 10-3
0.000 001 micro (µ) 10-6
0.000 000 001 nano (n) 10-9
0.000 000 000 001 pico (p) 10-12
0.000 000 000 000 001 femto (f) 10-15
0.000 000 000 000 000 001 atto (a) 10-18
0.000 000 000 000 000 000 001 zepto (z) 10-21
0.000 000 000 000 000 000 000 001 yocto (y) 10-24

Learning about the SI in schools

View and download our school-friendly posters which describe the SI and measurement in an easy to digest format.

School posters

Defining constants

Since 20 May 2019, the SI has been defined in terms of constants of nature, and is the system of units in which:

  • the unperturbed ground state hyperfine transition frequency of the caesium-133 atom Δν is 9 192 631 770 hertz, also known as the 'Cs frequency'
  • the speed of light in vacuum c is exactly 299 792 458 metres per second
  • the Planck constant h is exactly 6.626 070 15 × 10–34 joule seconds
  • the elementary charge e is exactly 1.602 176 634 × 10–19 coulombs
  • the Boltzmann constant k is exactly 1.380 649 × 10–23 joules per kelvin
  • the Avogadro constant NA is exactly 6.022 140 76 × 1023 reciprocal moles
  • the luminous efficacy of monochromatic radiation of frequency 540 ×1012 hertz Kcd, is exactly 683 lumens per watt

SI conventions

The following is a list of the key recommendations when using SI units:

Writing unit names and symbols

  • Only units of the SI and those units recognised for use with the SI should be used to express the values of quantities.
  • All unit names are written in small letters (newton or kilogram) except Celsius.
  • The unit symbol is in lower case unless the name of the unit is derived from a proper name, in which case the first letter of the symbol is in upper case.
  • Unit symbols are unaltered in the plural.
  • Unit symbols and unit names should not be mixed.
  • Abbreviations such as sec (for either s or second) or mps (for either m/s or metre per second) are not allowed.
  • For unit values more than 1 or less than -1 the plural of the unit is used and a singular unit is used for values between 1 and -1.
  • A space is left between the numerical value and unit symbol (25 kg, but not 25-kg or 25kg). If the spelled-out name of a unit is used, the normal rules of English are applied.
  • Unit symbols are in roman type, and quantity symbols are in italic type.

Numerical notation

  • A space should be left between groups of 3 digits on either the right or left hand side of the decimal place (15 739.012 53). However, when there are only four digits before or after the decimal marker, it is customary not to use a space to isolate a single digit. Commas should not be used.
  • The decimal marker shall be either the point on the line or the comma on the line. The decimal marker chosen should be that which is customary in the context concerned.
  • Mathematical operations should only be applied to unit symbols (kg/m2) and not unit names (kilogram/cubic metre).
  • Values of quantities should be expressed as 2.0 µs/s or 2.0 × 10-6 and not in terms such as parts per million.
  • It should be clear to which unit symbol a numerical value belongs and which mathematical operation applies to the value of a quantity (35 cm × 48 cm, not 35 × 48 cm; or 100 g ± 2 g, not 100 ± 2g).
SI derived units
Non-SI units accepted for use with the SI