The International System of Units (SI, Système international (d’unités)) consists of 7 base units
| Sl. No | Quantity | Definition | Unit |
| 1. | Time | The duration of 9192631770 periods of radiation, corresponds to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. | second |
| 2. | Length | The distance traveled by light in a vacuum in 1/299792458 seconds. | meter |
| 3. | Mass | The kilogram is defined by setting the Planck constant h exactly to 6.62607015×10−34 J⋅s (J = kg⋅m2⋅s−2), given the definitions of the meter and the second. | kilogram |
| 4. | Electric Current | The flow of exactly 1/1.602176634 x 10-19 times the elementary charge e per second. Equalling approximately 6.2415090744×1018 elementary charges per second. | ampere |
| 5. | Thermodynamic temperature | The kelvin is defined by setting the fixed numerical value of the Boltzmann constant k to 1.380649×10−23 J⋅K−1, (J = kg⋅m2⋅s−2), given the definition of the kilogram, the meter, and the second. | kelvin |
| 6. | Amount of substance | The amount of substance of exactly 6.02214076×1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol−1 | mole |
| 7. | Luminous Intensity | The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 5.4×1014 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. | candela |
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