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# The SI units

The globally agreed system of measurements 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 measurement of mass metre (m) Unit of measurement of length second (s) Unit of measurement of time ampere (A) Unit of measurement of electric current kelvin (K) Unit of measurement of thermodynamic temperature mole (mol) Unit of measurement of amount of substance candela (cd) Unit of measurement of luminous intensity

This International System of Units is necessary to ensure that our everyday concepts of measurement, whether a metre or a second, remain comparable and consistent worldwide. Standardising such measurements not only helps to keep them consistent and accurate, but also helps society have confidence in information. For instance, the kilogram is used every day, and defining this quantity means that consumers can trust that the shop is really providing the amount they say they are. Having reliable information on climate change, pollution and medical diagnostics is important to society and builds trust and allows effective decisions to be made.

See what the redefinition in 2019 will mean for the SI units

The impact on calibration services of the SI redefinition

## SI prefixes

SI prefixes are used to form decimal multiples and submultiples of SI units.

They should be used to avoid very large or very small numeric values.

The prefix attaches directly to the name of a unit, and a prefix symbol attaches directly to the symbol for a unit.

 Multiplying Factor SI Prefix 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 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

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