SI Unit

Definitions

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SI Units Definitions

Below are the definitions of each of the 7 SI base units. Further details with worked examples are available by clicking on the links under each unit definition as well as links to relevant calculators:

1. Meter (m) - Length: The meter is the fundamental unit of length in the SI system. It is defined by the distance traveled by light in a vacuum in: 1/299,792,458 seconds This definition links the unit of length to the speed of light, which is a fundamental constant of nature. [ More information with worked examples ] [ Speed - Distance - Time Calculator ]

2. Kilogram (kg) - Mass: The kilogram is the base unit of mass. It is defined by setting the Planck constant to be exactly: 6.62607015 x 10 –34 joule-seconds This definition ties the unit of mass to a fundamental physical constant. [ More information with worked examples ] [ Pounds to kilograms Calculator ]

3. Second (s) - Time: The second is the SI base unit of time. It is defined by the duration of: 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the cesium-133 atom at rest at a temperature of 0 K. This makes the second a very precise and stable time measurement. [ More information with worked examples ] [ Speed - Distance - Time Calculator ]

4. Ampere (A) - Electric Current: The ampere is the unit of electric current in the SI system. It is defined by setting the elementary charge to exactly: 1.602176634 x 10 –19 coulombs This definition connects the ampere to the charge of an electron, another fundamental constant. [ More information with worked examples ] [ Ohm’s Law Calculator ]

5. Kelvin (K) - Thermodynamic Temperature: The kelvin is the unit of thermodynamic temperature. It is defined by setting the Boltzmann constant to exactly: 1.380649 x 10 –23 joules per kelvin This links temperature to energy at the molecular level. [ More information with worked examples ] [ Celsius to Kelvin Calculator ]

6. Mole (mol) - Amount of Substance: The mole is the unit that measures the amount of substance. It is defined by setting the Avogadro constant to be exactly: 6.02214076 x 10 23 entities (e.g. particles) per mole This allows chemists and physicists to count atoms, molecules, and other particles using the mole as a convenient unit. [ More information with worked examples ]

7. Candela (cd) - Luminous Intensity: The candela is the SI unit of luminous intensity. It is defined by the luminous efficacy of monochromatic radiation of frequency: 540 x 10 12 hertz and has a radiant intensity of 1/683 watts per steradian This definition relates the candela to human perception of light intensity. [ More information with worked examples ] [ Candle power - cd Calculator ]

These base units are the foundation upon which all other SI units are built. Derived units are created by combining base units according to the rules of algebra. For example, the unit of force, the newton (N), is derived from the base units by the relationship 1 N = 1 kg·m/s². The SI system also includes a set of prefixes to indicate multiples and submultiples of the units, such as kilo- for a thousand times larger and milli- for a thousandth of the unit. [ More calculators - Power, Force, Speed/Distance, Gravity, Work Done etc. ]

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Common metric prefixes kilo (k): 1,000 (thousand) mega (M): 1,000,000 (million) giga (G): 1,000,000,000 (billion) Other metric prefixes are listed here.

SI Unit Definitions

Go to the SI Units Explained Page

SI Units Definitions

1. Meter (m) - Length: The meter is the fundamental unit of length in the SI system. It is defined by the distance traveled by light in a vacuum in: 1/299,792,458 seconds This definition links the unit of length to the speed of light, which is a fundamental constant of nature. More information with worked examples Speed - Distance - Time Calculator

2. Kilogram (kg) - Mass: The kilogram is the base unit of mass. It is defined by setting the Planck constant to be exactly: 6.62607015 x 10 –34 joule-seconds This definition ties the unit of mass to a fundamental physical constant. More information with worked examples Pounds to kilograms Calculator

3. Second (s) - Time: The second is the SI base unit of time. It is defined by the duration of: 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the cesium-133 atom at rest at a temperature of 0 K. This makes the second a very precise and stable time measurement. More information with worked examples Speed - Distance - Time Calculator

4. Ampere (A) - Electric Current: The ampere is the unit of electric current in the SI system. It is defined by setting the elementary charge to exactly: 1.602176634 x 10 –19 coulombs This definition connects the ampere to the charge of an electron, another fundamental constant. More information with worked examples Ohm’s Law Calculator

5. Kelvin (K) - Thermodynamic Temperature: The kelvin is the unit of thermodynamic temperature. It is defined by setting the Boltzmann constant to exactly: 1.380649 x 10 –23 joules per kelvin This links temperature to energy at the molecular level. More information with worked examples Celsius to Kelvin Calculator

7. Candela (cd) - Luminous Intensity: The candela is the SI unit of luminous intensity. It is defined by the luminous efficacy of monochromatic radiation of frequency: 540 x 10 12 hertz and has a radiant intensity of 1/683 watts per steradian. This definition relates the candela to human perception of light intensity. More information with worked examples Candle power - cd Calculator

These base units are the foundation upon which all other SI units are built. Derived units are created by combining base units according to the rules of algebra. For example, the unit of force, the newton (N), is derived from the base units by the relationship 1 N = 1 kg·m/s². The SI system also includes a set of prefixes to indicate multiples and submultiples of the units, such as kilo- for a thousand times larger and milli- for a thousandth of the unit. More calculators - Power, Force, Speed/Distance, Gravity etc.