# SI base unit

**base unitbase unitsbasebase quantitiesbasic unitfundamental SI unitsfundamental unitsSI base unitsSI unit systemSI units**

The International System of Units (SI) defines seven units of measure as a basic set from which all other SI units can be derived.wikipedia

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### International System of Units

**SISI unitsSI unit**

The International System of Units (SI) defines seven units of measure as a basic set from which all other SI units can be derived.

It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.

### SI derived unit

**derived unitderived unitsJ/kg**

The International System of Units (SI) defines seven units of measure as a 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).

### Kilogram

**kgmgmilligram**

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

The kilogram or kilogramme (symbol: kg) is the base unit of mass in the International System of Units (SI).

### Second

**ssecmegasecond**

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

The second is the SI base unit of time, 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.

### Ampere

**AmAamp**

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

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

### 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.

### Temperature

**temperaturesair temperaturewarm**

The most commonly used scales are the Celsius scale (formerly called centigrade) (denoted °C), Fahrenheit scale (denoted °F), and Kelvin scale (denoted K). The kelvin (spelled with a lower-case k) is the unit of temperature in the International System of Units (abbreviated SI), in which temperature is one of the seven fundamental base quantities.

### Kelvin

**KkelvinsmK**

The kelvin (symbol: K) is the base unit of temperature in the International System of Units (SI).

### Time

**temporaldurationintervals**

The SI base unit for time is the SI second.

### Length

**widthbreadthlengths**

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.

### Mole (unit)

**molemolmoles**

The mole is an SI base unit, with the unit symbol mol.

### Litre

**Lmlliter**

Several other units, such as the litre (US English: liter), are formally not part of the SI, but are accepted for use with SI.

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

### 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.

### Triple point

**triple point of waterhigh pressureTriple Point cell**

The triple point of water was used to define the kelvin, the base unit of thermodynamic temperature in the International System of Units (SI).

### Celsius

**°CCdegrees Celsius**

This definition also precisely relates 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. The temperature of the triple point of water is defined as exactly 273.16 K. This means that a temperature difference of one degree Celsius and that of one kelvin are exactly the same.

### Metrology

**metrologicalmetrologistlegal metrology**

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.

There is a proposed redefinition of the SI base units that was formally voted on in November 2018, and will come into effect in May 2019.

### 2019 redefinition of SI base units

**new definitionredefinition of SI base unitsproposed redefinition of SI base units**

In a note to the CIPM in October 2009, Ian Mills, the President of the CIPM Consultative Committee - Units (CCU) catalogued the uncertainties of the fundamental constants of physics according to the current definitions and their values under the proposed new definition.

A redefinition of SI base units is scheduled to come into force on 20 May 2019.

### Metre Convention

**Convention of the MetreTreaty of the Metre[61]**

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.

### Planck constant

**reduced Planck constantPlanck's constantreduced Planck's constant**

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

By 2017, the Planck constant had been measured with sufficient accuracy in terms of the SI base units, that it was central to replacing the metal cylinder, called the International Prototype of the Kilogram (IPK), that had defined the kilogram since 1889.

### Physical constant

**constantconstantsuniversal constant**

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.

On November 16, 2018, the International Bureau of Weights and Measures voted to redefine several base units in the International System of Units (SI) by fixing the SI value of several physical constants, including the Planck constant, h, elementary charge, e, Boltzmann constant, k B, Avogadro constant, N A, and the speed of light, c.

### Amount of substance

**amountamount of gasnumber of moles**

1972: The mole is approved as the SI base unit of amount of substance.

### Non-SI units mentioned in the SI

**accepted unit(Accepted for use with the SI)accepted for use with SI**

Several other units, such as the litre (US English: liter), are formally not part of the SI, but are accepted for use with SI.

SI base unit

### Metre

**metermmetres**

### Metric prefix

**prefixunit prefixdecimal**

Metric prefix

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