SI 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’.

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

In November 2022, measurement scientists and government representatives from around the world voted to expand the range of prefixes used within the International System of Units, meaning that four new prefixes, (which were proposed by NPL's Head of Metrology, Richard Brown) will now be used to express measurements worldwide. 

Find out more about the expansion of the SI prefixes here

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.

For the most up to date information on SI units, download the BIPM SI brochure

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.

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).

Derived units are defined as products of powers of the base units.

Twenty two of the coherent derived units in the SI are given special names (such as newton, pascal, joule, coulomb, volt, ohm etc). Together with the seven base units they form the core of the set of SI units.

All other SI units are combinations of some of these 29 units – some examples of derived units without special names are given in the table.

Examples of SI derived units expressed in terms of base units

There are certain non-SI units that are accepted for use with the SI. These include units which are in continuous everyday use, in particular the traditional units of time and of angle, together with a few other units which have assumed increasing technical importance and those needed for commercial, legal and specialist scientific interests or for the interpretation of older texts.

Non-SI units accepted for use with the International System

(a) As decided at the XXVIII General Assembly of the International Astronomical Union.

(b) For some applications such as in astronomy, small angles are measured in arcseconds (i.e. seconds of plane angle), denoted as or ″, or milliarcseconds, microarcseconds and picoarcseconds, denoted mas, μas and pas, respectively, where arcsecond is an alternative name for second of plane angle.

(c) The unit hectare and its symbol, ha, were adopted by the CIPM in 1879. The hectare is used to express land area.

(d) The litre and the symbol, lower-case l, were adopted by the CIPM in 1879. The alternative symbol, capital L, was adopted by the 16th CGPM in order to avoid the risk of confusion between the letter l (el) and the numeral 1 (one).

(e) The tonne and its symbol, t, were adopted by the CIPM in 1879. This unit is sometimes referred to as “metric ton” in some English-speaking countries.

(f) The dalton (Da) and the unified atomic mass unit (u) are alternative names (and symbols) for the same unit, equal to 1/12 of the mass of a free carbon 12 atom, at rest and in its ground state. This value of the dalton is the value recommended in the CODATA 2018 adjustment.

(g) The electronvolt is the kinetic energy acquired by an electron in passing through a potential difference of one volt in vacuum. The electronvolt is often combined with the SI prefixes.

(h) In using these units it is important that the nature of the quantity be specified and that any reference value used be specified.

This table, which is an extract of the 9^th edition of The International System of Units brochure published by BIPM, also includes the units of logarithmic ratio quantities, the neper, bel and decibel. The notes in the SI brochure provide more information on their use.

You can read more about the SI units and non-SI units in the full SI brochure.

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