The SI is maintained by a small agency in Paris, the International
Bureau of Weights and Measures ( At the heart of the SI is a concept called In the International System the base units are consistent with the part of the metric system called the MKS system. There are only seven SI base units: |

The other SI units, called SI derived
units, are then defined algebraically in terms of these base
units. For example, the SI unit of force, the newton,
is defined to be the force that accelerates a mass of one kilogram at
the rate of one meter per second per second. This means the newton is
equal to one kilogram meter per second squared, so the algebraic
relationship is N = kg·m·s^{-2}. Currently there
are 22 SI derived units that have special names and symbols. They include:

- the radian and steradian for plane and solid angles, respectively;
- the newton for force and the pascal for pressure;
- the joule for energy and the watt for power;
- the degree Celsius for everyday measurement of temperature;
- units for measurement of electricity: the coulomb (charge), volt (potential), farad (capacitance), ohm (resistance), and siemens (conductance);
- units for measurement of magnetism: the weber (flux), tesla (flux density), and henry (inductance);
- the lumen for flux of light and the lux for illuminance;
- the hertz for frequency of regular events and the becquerel for rates of radioactivity and other random events;
- the gray and sievert for radiation dose; and
- the katal, a unit of catalytic activity used in biochemistry.

Future meetings of the CGPM may make additions to this list; the katal was added by the 21st CGPM in 1999.

In addition to the 29 base and derived units, the SI permits the use of certain additional units, including:

- the traditional mathematical units for measuring angles (degree, arcminute, and arcsecond);
- the traditional units of civil time (minute, hour, day, and year);
- two metric units commonly used in ordinary life: the liter for volume and the tonne (metric ton) for large masses;
- the logarithmic units bel and neper (and their multiples, such as the decibel); and
- three non-metric scientific units whose values represent important physical constants: the astronomical unit, the atomic mass unit or dalton, and the electronvolt.

The SI currently accepts the use of certain other metric and non-metric units traditional in various fields. These units are supposed to be "defined in relation to the SI in every document in which they are used," and "their use is not encouraged." These barely-tolerated units could be prohibited by future meetings of the CGPM. They include:

- the nautical mile and knot, units traditionally used at sea and in meteorology;
- the are and hectare, common metric units of area;
- the bar, a pressure unit, and its commonly-used multiples such as the millibar in meteorology and the kilobar in engineering;
- the angstrom and the barn, units used in physics and astronomy.

The SI does not allow use of any units other than those listed above and their multiples. In particular, it does not allow use of any of the English traditional units (the horsepower, for example), nor does it allow the use of any of the algebraically-derived units of the former CGS system, such as the erg, gauss, poise, stokes, or gal. In addition, the SI does not allow use of other traditional scientific and engineering units, such as the torr, curie, calorie, or rem. Certain scientific fields have defined units more or less compatible with the SI, but not part of the SI. The use of the jansky in astronomy is a good example. There is always the chance that future meetings of the CGPM could add these units to the SI, but for the present they are not approved. |
The National Institute of Standards and Technology establishes measurement standards and presides over the implementation of the SI in the U.S. |

For multiples of approved units, the SI includes a list of prefixes.
This list has been extended several times, most recently by the 19th CGPM in
1991. Prefixes now range from yotta- at 10^{24} (one septillion) to
yocto- at 10^{-24} (one septillionth). The
SI does not allow these prefixes to be used for binary multiples, such as the
use of "kilobit" to mean 1024 bits instead of 1000. For binary
multiples a new list of special prefixes has been established by the International
Electrotechnical Commission.

Each SI unit is represented by a symbol, not an abbreviation. These symbols
are the same in every language of the world. However, the names of the units
themselves vary in spelling according to national conventions. Therefore, it
is correct for Americans to write *meter* and Germans to write *Meter*,
and it is also correct for the British to write *metre*, Italians to write
*metro*, and Poles to write *metr*. See Spelling
of Metric Units for additional comments.

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You are welcome to email the author (rowlett at email.unc.edu) with comments and suggestions.

All material in this folder is copyright 2018 by Russ Rowlett and the University of North Carolina at Chapel Hill. Permission is granted for personal use and for use by individual teachers in conducting their own classes. All other rights reserved. You are welcome to make links to this page, but please do not copy the contents of any page in this folder to another site. The material at this site will be updated from time to time.