The International System of Units is a decimal system of weights and measures which is based on and extends the metric system. It is abbreviated as SI in all languages.

SI includes seven basic units (see table 1). The metre, defined to be 1,650,763.73 wavelengths in vacuum of the red-orange line of the krypton-86 spectrum, is the SI unit for length. The kilogram, which is approximately 2.2 pounds avoirdupois and equals 1,000 grams (as defined by the platinum-iridium prototype kilogram held by the International Bureau of Weights and Measures in Sèvres, France), is the SI unit for mass. It is the sole base unit that remains defined by an artefact. It is also the only SI unit with a prefix as part of its name and symbol. The second, or the duration of 9,192,631,770 cycles of the radiation corresponding to a specified transition of the caesium-133 atom, is the SI unit for time. The ampere is the SI unit for electric current. It is the constant current produced by one volt which, when maintained in two parallel conductors separated by one metre in vacuum, generates an electromagnetic force of 2 x 10^-7 N m^-1. The kelvin, which is equal to 1/273.16 of the thermodynamic temperature at the triple point of water, is the SI unit for thermodynamic temperature. The kelvin’s magnitude equals that of the degree Celsius; however, a temperature expressed in degrees Celsius is the numerical equivalent of the temperature in kelvins less 273.15. The mole is the SI unit for amount of substance; it contains as many elementary units of substance as there are atoms in 0.012 kg of carbon-12. Elementary units must be specified, as they may be atoms, electrons, ions, molecules, radicals, etc. The candela is the SI unit for luminous intensity. It equals the luminous intensity of the black-body radiation, in the perpendicular direction, from an area of 1/600,000 square metres at platinum’s freezing temperature (2,042 kelvins) under 101,325 pascals of pressure, which approximates the intensity of a single paraffin candle.

Table 1. SI base units

¹ "Weight” is often used to mean “mass”.
² The name “degree kelvin” and the symbol “degK” were declared obsolete
at a 1967 international conference.

SI also includes two supplementary units (see table 2). Radian and steradian are the dimensionless units for the dimensionless quantities plane angle and solid angle, respectively. Units for other quantities are derived from the seven basic and two supplementary units.

Table 2. SI supplementary units

Table 3 lists selected SI derived units expressed in terms of base units. Derived units with special names and symbols are listed in table 4. These may be used to express other derived units (see table 5). The two supplementary units may also be used to express derived units (see table 6).

The 16 prefixes used to create multiples and submultiples of SI units are listed in table 7. Since multiple prefixes cannot be used, these prefixes are used with gram (g), but not with kilogram (kg).

A number of units that are not part of SI are widely used, especially in the United States. Those which are considered acceptable for use with SI in the US are listed in table 8. A conversion table for SI units is provided in table 9.

Table 3. Selected SI derived units expressed in terms of base units

Table 4. SI derived units with special names

¹ In addition to the thermodynamic temperature (T) expressed in kelvins (see table 105.1), Celsius
temperature (t) is also used and is defined by the equation t = T – T₀ where T₀ = 273.15 K by
definition. The unit “degree Celsius”, which is equal to the unit “kelvin”, is used to express Celsius
temperature.  Here, the term “degree Celsius” is a special name substituted for “kelvin”.
However, a difference or interval of Celsius temperature can be expressed in either kelvins
or degrees Celsius.

Table 5. Examples of SI derived units expressed with special names

Table 6. Examples of SI derived units formed with supplementary units

Table 7. SI prefixes

Table 8. Units in use with SI

¹ Both “l” and “L” are accepted as symbols for litre.
² In some countries, such as the United States, “metric ton” is used instead of “tonne”.
³ The values of these units in SI units are not known exactly; the values must be obtained
through experiment. The electronvolt is the kinetic energy acquired by an electron passing
through a potential difference of 1 volt in a vacuum.  The unified atomic mass unit equals 1/12 of
the mass of the atom of the nuclide ¹²C.

 Table 9. Conversion of non-SI units to SI units

Acknowledgement: The information in the tables is based primarily on data from the US National Institute of Standards and Technology (NIST).

Back