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# Physikalisch-Technische Bundesanstalt

Thematic toursGuardian of the units > The SI
The International System of Units (SI)

### The International System of Units (SI)

The introduction of the Système International d'Unités (with the international abbreviation SI) in 1960 put an end to the search for an internationally uniform system of units of measurement, which had lasted for centuries. Almost all states of the world today use the SI which comprises seven base units and many derived units.

### The seven SI base units

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

The derived SI units are coherently derived from the base units, which means that no conversion factors are required. Simple multiplication or division of base units is sufficient, and the same algebraic relations linking the corresponding quantities are used for the units. For example, the velocity is expressed as the ratio of the distance travelled to the time taken. Accordingly, the SI unit of velocity is equal to the quotient of the SI units of length and time: metre divided by second. Certain derived units have been given special names (e.g. hertz, newton, volt, ohm). They may themselves be used to form other derived units, and this in a simpler way than in terms of the base units.

When only the coherent SI units are used, the resulting numerical values of the quantities to be stated may be very large or very small. To ensure a practicable order of magnitude of the numerical values, prefixes have been adopted to express the decimal multiples and sub-multiples of the units.

### SI prefixes:

Factor Prefix Symbol Factor Prefix Symbol
1024 Yotta Y 10–1 Dezi d
1021 Zetta Z 10–2 Zenti c
1018 Exa E 10–3 Milli m
1015 Peta P 10–6 Mikro µ
1012 Tera T 10–9 Nano n
109 Giga G 10–12 Piko p
106 Mega M 10–15 Femto f
103 Kilo k 10–18 Atto a
102 Hekto h 10–21 Zepto z
101 Deka da 10–24 Yokto y