The Base Units of the Metric System

The difference between mass and weight is that weight reflects how gravity pulls on an object.
The difference between mass and weight is that weight reflects how gravity pulls on an object. On Earth, the values for mass and weight are the same. artpartner-images, Getty Images

The metric system is a system of units of measurement established from its beginnings in 1874 by diplomatic treaty to the more modern General Conference on Weights and Measures - CGPM (Conferérence Générale des Poids et Measures). The modern system is actually called the International System of Units or SI. SI is abbreviated from the French Le Système International d'Unités and grew from the original metric system. Today, most people use the named metric and SI interchangeably with SI being the correct title.

SI or metric is considered the main system of measurement units used in science today. Each unit is considered to be dimensionally independent of each other. These dimensions are described as the measurements of length, mass, time, electric current, temperature, amount of a substance, and luminous intensity. This list has the current definitions of each of the seven base units.

  • Length - Meter or Metre (m)The meter is the SI unit of length. The meter is defined by the length of the path light travels in a vacuum during 1/299 792 458 of a second.
  • Mass - Kilogram (kg)
    The kilogram is the SI unit of mass. It is the mass of the international prototype of the kilogram. There is a standard platinum/iridium 1 kg mass housed near Paris at the International Bureau of Weights and Measures (BIPM).
  • Time - Second (s)
    The basic unit of time is the second. The second is defined to be the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the cesium-133 atomic ground state.
  • Electric Current - Ampere (A)
    The basic unit of electric current is the ampere. The ampere is defined to be that constant current which, if maintained in two infinitely long straight parallel conductors which have a negligible circular cross-section, and placed 1 m apart in vacuum, would produce between a force between the conductors equal to 2 x 10-7 newton per meter of length.
  • Temperature - Kelvin (K)
    The Kelvin is the unit of thermodynamic temperature. It is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The Kelvin scale is an absolute scale, so there is no degree.​
  • Amount of a Substance - Mole (mol)
    The mole is defined to be the amount of a substance which contains as many entities as there are atoms in 0.012 kilograms of carbon-12. When the mole unit is used, the entities must be specified. For example, the entities may be atoms, molecules, ions, electrons, cows, houses, or anything else.
  • Luminous Intensity - candela (cd)
    The unit of luminous intensity, or light, is the candela. The candela is the luminous intensity, in a given direction, of a source emitting monochromatic radiation of frequency 540 x 1012 hertz with radiant intensity in that direction of 1/683 watt per steradian.

These definitions are actually methods to realize the unit. Each realization was created with a unique and sound theoretical base to generate reproducible and accurate results.

Important Non-SI Units

In addition to the seven base units, some non-SI units are commonly used:

  • Liter (L) - While the SI unit of volume is the cubic meter, m3, the most commonly used unit is the liter. A liter is equal in volume to one cubic decimeter, dm 3, which is a cube that is 0.1 m on each side.
  • Angstrom (Å) - One Angstrom equals 10-8 cm or 10-10 m. Named for Anders Jonas Ångstrom, the unit is used to measure chemical bond length and electromagnetic radiation wavelength.
  • Cubic Centimeter (cm3) - A cubic centimeter is a common unit used to measure solid volume. The corresponding unit for liquid volume is the milliliter (mL), which is equal to one cubic centimeter.