The International System of Units is the modern form of the metric system and is generally a system devised around the convenience of the number ten.

It is the world's most widely used system of measurement, both in everyday commerce and in science. The older metric system included several groups of units. The SI was developed in 1960 from the old metre-kilogram-second system.  Because the SI is not static, units are created and definitions are modified through international agreement among many nations as the technology of measurement progresses, and as the precision of measurements improves.

The system is nearly universally employed. Three principal exceptions are Burma (Myanmar), Liberia, and the United States. The United Kingdom has officially adopted the International System of Units but not with the intention of replacing customary measures entirely.


There are seven base units of the SI:

  • METRE (m) - The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
  • KILOGRAM (kg) - The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.
  • SECOND (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 caesium 133 atom.
  • AMPERE (A) - The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length.
  • KELVIN (K) - The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
  • MOLE (mol) – The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.
  • CANDELA (cd) - The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.


A prefix may be added to a unit to produce a multiple of the original unit. All multiples are integer powers of ten.

Standard prefixes for the SI units of measure

Standard prefixes for the SI units of measure

SI derived unit

The International System of Units (SI) specifies a set of seven base units from which all other units of measurement are formed. These units are called SI derived units and are also considered part of the standard.

SI derived units


Other common units, such as the litre, are not SI units, but are accepted for use with SI

Compound units derived from SI units

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The Greek alphabet is a set of twenty-four letters that has been used to write the Greek language since the late 9th or early 8th century BCE. It is the first and oldest alphabet in the narrow sense that it notes each vowel and consonant with a separate symbol. It is as such in continuous use to this day. The letters were also used to represent Greek numerals, beginning in the 2nd century BCE.

Greek symbols are traditionally used as names in mathematics, physics and other sciences. When combined with Latin characters, the Latin characters usually indicate variables while the Greek ones indicate parameters. Many symbols have traditional meanings, such as lower case alpha (α) for angle of attack in fluid dynamics, lower case epsilon (ε) for `a small number, which tends towards the infinitesimal', capital sigma (Σ) for `sum', and lower case sigma (σ) for standard deviation.

Greek alphabet

Submitted by Ceh Jan