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### International System of Units

**SISI unitsSI unit**

The SI base units are seven units of measure defined by the International System of Units as the basic set from which all other SI units can be derived.

The base units are defined in terms of invariant constants of nature, such as the speed of light in vacuum and the charge of the electron, which can be observed and measured with great accuracy.

### SI derived unit

**derived unitderived unitsJ/kg**

The SI base units are seven units of measure defined by the International System of Units as the basic set from which all other SI units can be derived.

SI derived units are units of measurement derived from the seven base units specified by the International System of Units (SI).

### Second

**ssecmegasecond**

The units and their physical quantities are the second for time, the metre for measurement of length, the kilogram for mass, the ampere for electric current, the kelvin for temperature, the mole for amount of substance, and the candela for luminous intensity.

The second (symbol: s, abbreviation: sec) is the base unit of time in the International System of Units (SI), commonly understood and historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each.

### Kilogram

**kgmgmilligram**

The units and their physical quantities are the second for time, the metre for measurement of length, the kilogram for mass, the ampere for electric current, the kelvin for temperature, the mole for amount of substance, and the candela for luminous intensity.

The kilogram (also kilogramme) is the base unit of mass in the metric system, formally the International System of Units (SI), having the unit symbol kg.

### Ampere

**AmAamp**

The units and their physical quantities are the second for time, the metre for measurement of length, the kilogram for mass, the ampere for electric current, the kelvin for temperature, the mole for amount of substance, and the candela for luminous intensity.

The ampere ( or (UK), symbol: A), often shortened to "amp", is the base unit of electric current in the International System of Units (SI).

### Kelvin

**KkelvinsKelvin scale**

The kelvin is the base unit of temperature in the International System of Units (SI), having the unit symbol K. It is named after the Belfast-born, Glasgow University engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907).

### Mole (unit)

**molemolmoles**

The mole (symbol: mol) is the base unit of amount of substance ("number of substance") in the International System of Units or System International (SI), defined as containing exactly 6.02214076 particles, e.g., atoms, molecules, ions or electrons.

### Candela

**cdCandelasmegacandela**

The candela ( or ; symbol: cd) is the base unit of luminous intensity in the International System of Units (SI); that is, luminous power per unit solid angle emitted by a point light source in a particular direction.

### Time

**temporaldurationsequence of events**

The SI base unit for time is the SI second.

### Unit of measurement

**unitunits of measurementweights and measures**

The SI base units are seven units of measure defined by the International System of Units as the basic set from which all other SI units can be derived.

In this SI units system, there are seven SI base units and three supplementary units.

### Litre

**Lmlliter**

A number of other units, such as the litre (US English: liter), astronomical unit and electronvolt, are not formally part of the SI, but are accepted for use with SI.

The original French metric system used the litre as a base unit.

### Length

**widthlengthsbreadth**

In the International System of Units (SI), the basic unit of length is the metre and is now defined in terms of the speed of light.

### 2005–2019 definitions of the SI base units

**preceding definitions of the SI base units**

On 20 May 2019, as the final act of the 2019 redefinition of the SI base units, the BIPM officially introduced the following new definitions, replacing the preceding definitions of the SI base units.

From 2005 to early 2019, the definitions of the SI base units were as follows:

### Luminous intensity

**intensityluminouslight intensity**

The SI unit of luminous intensity is the candela (cd), an SI base unit.

### Day

**ddiurnaldays**

In 1960, the second was redefined in terms of the orbital motion of the Earth in year 1900, and was designated the SI base unit of time.

### Celsius

**°CCdegrees Celsius**

This definition also precisely related the Celsius scale to the Kelvin scale, which defines the SI base unit of thermodynamic temperature with symbol K. Absolute zero, the lowest temperature possible, is defined as being exactly 0 K and −273.15 °C.

### Amount of substance

**amountamount of gasamount of a substance**

### 2019 redefinition of the SI base units

**2019 redefinition of SI base unitsredefinitionredefinition of the SI base units**

On 20 May 2019, as the final act of the 2019 redefinition of the SI base units, the BIPM officially introduced the following new definitions, replacing the preceding definitions of the SI base units.

In 2019, the SI base units were redefined, effective on 144th anniversary of the Metre Convention, 20 May 2019.

### Non-SI units mentioned in the SI

**accepted for use with SIAccepted for use with the SIaccepted unit**

A number of other units, such as the litre (US English: liter), astronomical unit and electronvolt, are not formally part of the SI, but are accepted for use with SI.

### Metre

**metermmetres**

### Metric prefix

**SI prefixunit prefixprefix**

This also applies to mass, for which the SI base unit (kilogram) already contains a prefix.

### Metre Convention

**Treaty of the MetreConvention of the MetreConvention du Mètre**

The definitions of the base units have been modified several times since the Metre Convention in 1875, and new additions of base units have occurred.

On 16 November 2018, the 26th General Conference on Weights and Measures (CGPM) voted unanimously in favour of revised definitions of some SI base units, in particular the kilogram.

### Avogadro constant

**Avogadro's numberAvogadro numberAvogadro’s number**

Two possibilities attracted particular attention: the Planck constant and the Avogadro constant.

In 1971 the International Bureau of Weights and Measures (IBPM) decided to regard the amount of substance as an independent dimension of measurement, with the mole as its base unit in the International System of Units (SI).

### General Conference on Weights and Measures

**CGPMConférence Générale des Poids et MesuresCGPM conference**

The 21st General Conference on Weights and Measures (CGPM, 1999) placed these efforts on an official footing, and recommended "that national laboratories continue their efforts to refine experiments that link the unit of mass to fundamental or atomic constants with a view to a future redefinition of the kilogram".

### Physical constant

**constantconstantsfundamental constants**

It has long been an objective in metrology to define the kilogram in terms of a fundamental constant, in the same way that the metre is now defined in terms of the speed of light.

Similarly, with effect from May 2019, the Planck constant has a defined value, such that all SI base units are now defined in terms of fundamental physical constants.