The 7 Base Units of the Metric System

The units are designed to be reproducible and accurate

five weights of different sizes

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The metric system is a framework of units of measurement that has grown from its 1874 birth in a diplomatic treaty to the more modern General Conference on Weights and Measures, or CGPM (Conferérence Générale des Poids et Measures). The modern system is properly called the International System of Units, or SI, an abbreviation from the French Le Système International d'Unités. Today, most people use the names metric and SI interchangeably.

The 7 Base Metric Units

The metric system is the main system of measurement units used in science. Each unit is considered to be dimensionally independent of the others. These dimensions are measurements of length, mass, time, electric current, temperature, amount of a substance, and luminous intensity. Here are definitions of the seven base units:

  • Length: Meter (m) The meter is the metric unit of length. It's defined as 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 metric unit of mass. It's the mass of the international prototype of the kilogram: 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 as the duration of 9,192,631,770 oscillations of radiation corresponding to the transition between the two hyperfine levels of cesium-133.
  • Electric current: Ampere (A) The basic unit of electric current is the ampere. The ampere is defined as the constant current that, if maintained in two infinitely long straight parallel conductors with a negligible circular cross-section and placed 1 m apart in a vacuum, would produce a force between the conductors equal to 2 x 10-7 newtons 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 as the amount of a substance that 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, sound theoretical base to generate reproducible and accurate results.

Other Important Metric Units

In addition to the seven base units, other metric units are commonly used:

  • Liter (L) While the metric 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 the 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.