# Outline of the metric system

"The metric system is for all people for all time." (Condorcet 1791) Four objects used in making measurements in everyday situations that have metric calibrations are shown: a tape measure calibrated in centimetres, a thermometer calibrated in degrees Celsius, a kilogram mass, and an electrical multimeter which measures volts, amps and ohms.

The following outline is provided as an overview of and topical guide to the metric system – various loosely related systems of measurement that trace their origin to the decimal system of measurement introduced in France during the French Revolution.

## Nature of the metric system

The metric system can be described as all of the following:

• System – set of interacting or interdependent components forming an integrated whole.
• System of measurement – set of units which can be used to specify anything which can be measured. Historically, systems of measurement were initially defined and regulated to support trade and internal commerce. Units were arbitrarily defined by fiat (see statutory law) by the ruling entities and were not necessarily well inter-related or self-consistent. When later analyzed and scientifically, some quantities were designated as base units, meaning all other needed units of measure could be derived from them.

### Underlying philosophy

Discussions of the underlying philosophy of the metric system (and other systems of measure) include:

## Metric units of measure

Articles that exist for many units of measure that are related to various flavours of the metric system are catalogued below.

Name Quantity SI unit cgs unit Other metric Non-metric
abampere electric current Derived metric unit with a special name
abcoulomb electric charge Derived metric unit with a special name
abhenry inductance Derived metric unit with a special name
abohm electrical resistance Derived metric unit with a special name
abvolt potential difference Derived metric unit with a special name
ampere electric current Base unit
ampere-meter magnetic pole strength Derived coherent metric unit that has no special name
apostilb luminance Derived metric unit with a special name
astronomical unit length Non-coherent unit explicitly permitted for use within the metric system
dalton mass Non-coherent unit explicitly permitted for use within the metric system
barye pressure Derived metric unit with a special name
becquerel radioactive activity Derived metric unit with a special name
bril luminance No specified relationship
candela per square metre luminance Derived coherent metric unit that has no special name
candela luminous intensity Base unit
degree Celsius temperature Derived metric unit with a special name
centimetre length Multiple or submultiple of a base unit Base unit
coulomb electric charge Derived metric unit with a special name
cubic centimetre volume Derived coherent metric unit that has no special name Derived coherent metric unit that has no special name
cubic metre per second volumetric flow rate Derived coherent metric unit that has no special name Derived coherent metric unit that has no special name
cubic metre volume Derived coherent metric unit that has no special name Derived coherent metric unit that has no special name
curie radioactive activity Derived metric unit with a special name
day time Non-coherent unit explicitly permitted for use within the metric system
decibel level Non-coherent unit explicitly permitted for use within the metric system
degree of arc angle Non-coherent unit explicitly permitted for use within the metric system
dyne force Derived metric unit with a special name
electronvolt energy Non-coherent unit explicitly permitted for use within the metric system
erg energy Derived metric unit with a special name
farad capacitance Derived metric unit with a special name
gal acceleration Derived metric unit with a special name
gauss magnetic flux density Derived metric unit with a special name
gram mass Multiple or submultiple of a base unit Base unit
grave mass Base unit
gray absorbed dose Derived metric unit with a special name
hectare area Non-coherent unit explicitly permitted for use within the metric system Multiple or submultiple of a base unit
henry inductance Derived metric unit with a special name
hertz frequency Derived metric unit with a special name Derived metric unit with a special name
hour time Non-coherent unit explicitly permitted for use within the metric system No specified relationship
joule per mole energy per amount of substance Derived coherent metric unit that has no special name
joule energy Derived metric unit with a special name
joule-second angular momentum Derived coherent metric unit that has no special name
katal catalytic activity Derived metric unit with a special name
kelvin temperature Base unit Base unit
kilogram per cubic metre density Derived coherent metric unit that has no special name Derived coherent metric unit that has no special name
kilogram mass Base unit Multiple or submultiple of a base unit
kilometres per hour velocity No specified relationship
litre volume Non-coherent unit explicitly permitted for use within the metric system
lumen luminous flux Derived metric unit with a special name
lumen second luminous energy Derived coherent metric unit that has no special name
lux second luminous exposure Derived coherent metric unit that has no special name
lux illuminance Derived metric unit with a special name
maxwell magnetic flux Derived metric unit with a special name
metre per second squared acceleration Derived coherent metric unit that has no special name Derived coherent metric unit that has no special name
metre squared per second angular momentum Derived coherent metric unit that has no special name Derived coherent metric unit that has no special name
metre length Base unit Multiple or submultiple of a base unit
microgram mass Multiple or submultiple of a base unit Multiple or submultiple of a base unit
minute of arc angle Non-coherent unit explicitly permitted for use within the metric system
minute time Non-coherent unit explicitly permitted for use within the metric system No specified relationship
mole amount of substance Base unit
neper level Non-coherent unit explicitly permitted for use within the metric system
newton force Derived metric unit with a special name
newton metre torque Derived coherent metric unit that has no special name
newton-second impulse or momentum Derived coherent metric unit that has no special name
oersted magnetic field strength Derived metric unit with a special name
ohm electric resistance Derived metric unit with a special name
pascal pressure Derived metric unit with a special name
phot illuminance Derived coherent metric unit that has no special name
poise dynamic viscosity Derived metric unit with a special name
radian per second angular frequency Derived coherent metric unit that has no special name
radian angle Derived metric unit with a special name
rayleigh photon flux No specified relationship
roentgen kerma of X-rays and gamma rays Derived coherent metric unit that has no special name
roentgen equivalent man radiation dose equivalent Derived coherent metric unit that has no special name
second time Base unit Base unit
siemens electric conductance Derived metric unit with a special name
sievert radiation dose equivalent Derived metric unit with a special name
skot luminance No specified relationship
square kilometre area Derived coherent metric unit that has no special name Derived coherent metric unit that has no special name
square metre area Derived coherent metric unit that has no special name Derived coherent metric unit that has no special name
statcoulomb electric charge Derived metric unit with a special name
statvolt potential difference Derived metric unit with a special name
steradian solid angle Derived metric unit with a special name
stilb luminance Derived coherent metric unit that has no special name
torr pressure No specified relationship
stokes kinematic viscosity Derived metric unit with a special name
tesla magnetic field strength Derived metric unit with a special name
tonne mass Non-coherent unit explicitly permitted for use within the metric system
volt potential difference Derived metric unit with a special name
watt second energy Derived coherent metric unit that has no special name
watt power Derived metric unit with a special name
weber magnetic flux Derived metric unit with a special name

## History of the metric system

History of the metric system – the metric system developed from a decimal system of measurement adopted by France after the French Revolution.

### Chronological history of the metric system

Principal dates in the development of the metric system include:[1]

• 1792 – Initiation of a decimal system of measurement by the French Revolutionary Government
• 1799 – The Metre des archives and kilogram des archives become the standards for the metric system.
• 1861 – Concept of unit coherence introduced by Maxwell – the base units were the centimetre, gram and second.
• 1875 – Under the Convention of the metre, a new body the General Conference on Weights and Measures (CGPM) was set up and given responsibility for the new international prototype kilogram and international prototype metre which replaced the old French copies as the definitive prototype metre and kilogram.
• 1881 – International Electric Congress agrees on standards for electrical units (formalised in 1893)
• 1921 – Convention of the metre extended to cover all physical units of measure
• 1960 – The CGPM published the metric system under the name "International System of Units" (SI) a coherent system of units based on the kilogram, metre, second, ampere and kelvin.

### History of metrication

History of metrication – metrication is the process by which legacy, national-specific systems of measurement were replaced by the metric system.

### Historical metric system variants

Four variants of the metric system that predate the introduction of SI (1960) are described in varying levels of detail:

Between 1812 and 1839 France used a quasi-metric system:

## Politics of the metric system

Prior to 1875 the metric system was controlled by the French Government. In that year, seventeen nations signed the Metre Convention and the management and administration of the system passed into international control.

Both the European Union and the International Organization for Standardization have issued directives/recommendations to harmonise the use of units of measure. These documents endorse the use of SI for most purposes.