The metric system is an internationally recognized decimalized system of measurement. It is in widespread use, and where it is adopted, it is the only or most common system of weights and measures. It is now known as the International System of Units. All countries in the world use it except the United States, Myanmar (Burma) and Liberia.

## Base units:

- The meter for length
- kilogram for mass
- second for time
- ampere for electromagnetism
- Kelvin for temperature
- candela for luminous intensity
- mole for quantity

For example, The speed of light is defined as 299,792,458 meters per second

## Eight of these units are electromagnetic quantities:

- Volt, a unit of electrical potential
- Ohm, a unit of electrical resistance
- Tesla, a unit of magnetic flux density
- Weber, a unit of magnetic flux
- Farad, a unit of electrical capacitance
- Henry, a unit of electrical inductance
- Siemens, a unit of electrical conductance (the inverse of an ohm)
- Coulomb, a unit of electrical charge

## Four of these units are mechanical quantities:

- Watt, a unit of mechanical or electrical power
- Newton, a unit of mechanical force
- Joule, a unit of mechanical, electrical or thermodynamic energy
- Pascal, a unit of pressure

## Five units represent measures of electromagnetic radiation and radioactivity:

- Becquerel, a unit of radioactive decay
- Sievert, a unit of absorbed ionizing radiation
- Gray, a unit of ionizing radiation
- Lux, a unit of luminous flux
- Lumen, a unit of luminous intensity

## Two units are measures of circular arcs and spherical surfaces:

- Radian, a unit of a circular arc
- Steradian, a unit of spherical surface area

## Three units are miscellaneous:

- Degree Celsius, a unit of thermodynamic temperature
- Katal, a unit of catalytic activity (enzymatic)
- Hertz, a unit of cycles per second (inverse of a second)