SI Base Units

The SI base units are standard units of measurement in the International System of Units (SI) for seven base quantities. All other SI units are derived from this simple set of seven units. Here is a list of the SI base units, their abbreviations, and a look at their current and past definition.

The Seven SI Base Units

Here are the seven SI base units and their abbreviations:

Note that the names of all of the SI base units are written in lowercase letters, even when they honor a person. The symbols, however, are lowercase letters except when they honor a person. So, the symbol for the kelvin is K because it takes its name from Lord Kelvin and the ampere has the symbol A because it is named for André-Marie Ampère.

SI Unit Definitions – Past and Present

On May 20, 2019, the International Bureau of Weights and Measures (BIPM) officially re-defined the seven base units. The reason is so all of the base unit definitions come from a property of nature (which remain constant) and not some physical object (which could change over time).

• Second: The second is based on an atomic clock. It is the duration of 9,192,631,770 periods of transition between the two hyperfine levels of the ground state of the caesium-133 atom at 0 K. Previously, the was defined according to the mean solar day of 24 hours, where each hour has 60 minutes and each minute has 60 seconds. In other words, the second was defined as ¹⁄₈₆₄₀₀ of one day. As it turns out, each day isn’t the same length, so the original definition wasn’t as constant as you might think.
  
  
• Meter: The meter (or metre, depending where you live) depends on the definition of the second and the speed of light in a vacuum, which is a constant. The meter is the distance travelled by light in a vacuum in ¹/_299792458 of a second. The original meter definition in 1793 was one ten-millionth of the distance from the equator to the North Pole, travelling along a great circle. Since the Earth isn’t actually round, much less smooth, the definition wasn’t the best. Later, the definition depended on a prototype meter bar and then on a number of wavelengths of an emission line of the noble gas krypton-86.
  
  
• Kilogram: The modern kilogram definition depends on Planck’s constant, the meter definition, and the second definition. It is the mass from Planck’s constant h, when expressed in the units kg m² s^-1, where the meter and second are defined and Planck’s constant is fixed at 6.62607015×10⁻³⁴ J s or kg m² s^-1. The original kilogram definition was the mass of one liter of water at the melting point of water, which was liter was one thousandth of a cubic meter.
  
  
• Ampere: The ampere definition depends on the charge of the electron. The electron charge is a fixed value of 1.602176634×10⁻¹⁹ C, where a coulomb C is an ampere second A s. The original definition was the electrical current required to deposit 1.118 milligrams of silver per second from silver nitrate. Later, the definition changed such that it was the constant current between two parallel conductors one meter apart in a vacuum that produced a force of 2 x 10^-7 newton’s per meter.
  
  
• Kelvin: The kelvin definition depends on the Boltzmann constant and the definitions of the meter, kilogram, and second. It is the temperature obtained when the Boltzmann constant k is expressed in J K^-1, which is the same as kg m² s^-2 K^-1. The Boltzmann constant is 1.380649×10⁻²³ J/K. Originally, the kelvin was temperature using the same size increment as the Celsius degree, but starting at absolute zero.
  
  
• Mole: The mole depends on the definition of Avogadro’s constant, which is exactly 6.022 140 76 × 10²³ particles. Avogadro’s number is the value of Avogadro’s constant expressed in the mol^-1 unit. The earlier definition depended on an empirical measurement of the number of atoms in 0.012 kilograms of the isotope carbon-12.
  
  
• Candela: The candela depends on the light emitted by a green light of a specified power and the kilogram, meter, and second definitions. The illumination from monochromatic light with a frequency of 540 × 10¹² Hz (about 555 nm) is 683 lm W^-1, which is the same unit as cd sr kg^-1 m^-2 s³. The original candela unit comes from the candlepower, which is the light emitted by a burned candle under standard conditions.

Derived Units

There are 22 derived units with special names and around 100 other units. The derived units are multiples and combinations of the SI base units.

References

• Bureau International des Poids et Mesures (2019). The International System of Units (SI) (9th ed.).