Since its introduction in 1960, the base units for the International system of units, known as SI, have changed several times. Tables in this article summarize those changes.
When Maxwell first introduced the concept of a coherent system, he identified three quantities that could be used as base units: mass, length, and time. Giorgi later identified the need for an electrical base unit, for which the unit of electric current was chosen for SI. Another three base units (for temperature, amount of substance, and luminous intensity) were added later.^{[1]}
The early metric systems defined a unit of weight as a base unit, while the SI defines an analogous unit of mass. In everyday use, these are mostly interchangeable, but in scientific contexts the difference matters. Mass, strictly the inertial mass, represents a quantity of matter. It relates the acceleration of a body to the applied force via Newton's law, F = m × a: force equals mass times acceleration. A force of 1 N (newton) applied to a mass of 1 kg will accelerate it at 1 m/s^{2}. This is true whether the object is floating in space or in a gravity field e.g. at the Earth's surface. Weight is the force exerted on a body by a gravitational field, and hence its weight depends on the strength of the gravitational field. Weight of a 1 kg mass at the Earth's surface is m × g; mass times the acceleration due to gravity, which is 9.81 newtons at the Earth's surface and is about 3.5 newtons at the surface of Mars. Since the acceleration due to gravity is local and varies by location and altitude on the Earth, weight is unsuitable for precision measurements of a property of a body, and this makes a unit of weight unsuitable as a base unit.^{[citation needed]}
Since 1960 the CGPM has made a number of changes to the SI to meet the needs of specific fields, notably chemistry and radiometry. These are mostly additions to the list of named derived units, and include the mole (symbol mol) for an amount of substance, the pascal (symbol Pa) for pressure, the siemens (symbol S) for electrical conductance, the becquerel (symbol Bq) for "activity referred to a radionuclide", the gray (symbol Gy) for ionising radiation, the sievert (symbol Sv) as the unit of dose equivalent radiation, and the katal (symbol kat) for catalytic activity.^{[2]}^{: 156, 158–159, 165 }^{[3]}^{: 221 }
The range of defined prefixes pico (10^{−12}) to tera (10^{12}) was extended to quecto (10^{−30}) to quetta (10^{30}).^{[2]}^{: 152, 158, 164 }
The 1960 definition of the standard metre in terms of wavelengths of a specific emission of the krypton86 atom was replaced in 1983 with the distance that light travels in vacuum in exactly 1/299792458 second, so that the speed of light is now an exactly specified constant of nature.^{[citation needed]}
A few changes to notation conventions have also been made to alleviate lexicographic ambiguities. An analysis under the aegis of CSIRO, published in 2009 by the Royal Society, has pointed out the opportunities to finish the realisation of that goal, to the point of universal zeroambiguity machine readability.^{[4]}
Unit name  Definition^{[n 1]} 

second 

metre 

kilogram 

ampere 

kelvin 

mole 

candela 

The Prior definitions of the various base units in the above table were made by the following authors and authorities:
All other definitions result from resolutions by either CGPM or the CIPM and are catalogued in the SI Brochure. 
Prior to the 2019 redefinition of SI base units, from 2005 to early 2019, the SI base units were defined as follows.
Name  Symbol  Measure  Pre2019 (2005) formal definition^{[2]}  Historical origin / justification  Dimension symbol 

metre  m  length  "The metre is the length of the path travelled by light in vacuum during a time interval of 1 / 299792458 of a second." 17th CGPM (1983, Resolution 1, CR, 97) 
1 / 10000000 of the distance from the Earth's equator to the North Pole measured on the circumference through Paris.  L 
kilogram  kg  mass  "The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram." 3rd CGPM (1901, CR, 70) 
The mass of one litre of water at the temperature of melting ice. A litre is one thousandth of a cubic metre.  M 
second  s  time  "The second is the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom." 13th CGPM (1967/68, Resolution 1; CR, 103) "This definition refers to a caesium atom at rest at a temperature of 0 K." (Added by CIPM in 1997) 
The day is divided in 24 hours, each hour divided in 60 minutes, each minute divided in 60 seconds. A second is 1 / (24 × 60 × 60) of the day. 
T 
ampere  A  electric current  "The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular crosssection, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2×10^{−7} newton per metre of length." 9th CGPM (1948) 
The original "Absolute Ampere" was defined as 0.1 Electromagnetic units. The original "International Ampere" was defined electrochemically as the current required to deposit 1.118 milligrams of silver per second from a solution of silver nitrate. Compared to the SI ampere, the difference is 0.015%. 
I 
kelvin  K  thermodynamic temperature  "The kelvin, unit of thermodynamic temperature, is the fraction 1 / 273.16 of the thermodynamic temperature of the triple point of water." 13th CGPM (1967/68, Resolution 4; CR, 104) "This definition refers to water having the isotopic composition defined exactly by the following amount of substance ratios: 0.000 155 76 mole of ^{2}H per mole of ^{1}H, 0.000 379 9 mole of ^{17}O per mole of ^{16}O, and 0.002 005 2 mole of ^{18}O per mole of ^{16}O." (Added by CIPM in 2005) 
The Celsius scale: the Kelvin scale uses the degree Celsius for its unit increment, but is a thermodynamic scale (0 K is absolute zero).  Θ 
mole  mol  amount of substance  "1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is 'mol'. 2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles." 14th CGPM (1971, Resolution 3; CR, 78) "In this definition, it is understood that unbound atoms of carbon 12, at rest and in their ground state, are referred to." (Added by CIPM in 1980) 
Atomic weight or molecular weight divided by the molar mass constant, 1 g/mol.  N 
candela  cd  luminous intensity  "The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×10^{12} hertz and that has a radiant intensity in that direction of 1 / 683 watt per steradian." 16th CGPM (1979, Resolution 3; CR, 100) 
The candlepower, which is based on the light emitted from a burning candle of standard properties.  J 
Name  Symbol  Measure  Pre2019 (2005) formal definition^{[2]}  Historical origin / justification  Dimension symbol 