# Basic, derived SI units, prefixes and constants

In Europe, the units for measuring different properties are known as SI units. SI stands for the * International System of Units (Système International d’Unités, abbreviated SI)*. It comprises a coherent system of units of measurement starting with seven base units. The SI unit system is informally known as the metric system. Scroll down and find out all about basic, derived SI units, prefixes and constants.

All units are derived from the seventh base units.

#### SI units

Base quantity | Base unit | Symbol |

Time | Second | s |

Electrical current | Ampere | A |

Length | Metre | m |

Mass | Kilogram | kg |

Temperature | Kelvin | K |

Luminous intensity | Candela | cd |

Amount of substance | Mole | mol |

The SI electrical units are based on the mksa (meter-kilogram-second-ampere) system. They have been adopted by the standardization bodies of the world, including the International Electrotechnical Commission (IEC), the American National Standards Institute (ANSI), and the Standards Board of the Institute of Electrical and Electronics Engineers (IEEE). The United States is the only industrialized nation in the world that does not mandate the use of the SI system.

Although the U.S. Congress has the constitutional right to establish measuring units, it has never enforced any system. The metric system (now SI) was legalized by Congress in 1866 and is the only legal measuring system, but other non-SI units are legal as well.

The system allows for an unlimited number of additional units, called derived units, which can always be represented as products of powers of the base units.

#### Derived SI units

Derived quantity | Name | Symbol |

Frequency | Hertz | Hz |

Force | Newton | N |

Energy, work, quantity of heat | Joule | J |

Electric charge, quantity of electricity | Coulomb | C |

Power | Watt | W |

Potential difference, electromotive force | Volt | V or U |

Capacitance | Farad | F |

Electrical resistance | Ohm | Ω |

Magnetic flux | Weber | Wb |

Inductance | Henry | H |

Luminous flux | Lumen | cd |

Area | Square metre | m^{2} |

Volume | Cubic metre | m³ |

Velocity, speed | Metre per second | m/s |

Mass density | Kilogram per cubic metre | kg/m³ |

Luminance | Candela per square metre | cd/m^{2} |

## SI unit prefixes

Like all metric systems, the SI uses metric prefixes to systematically construct, for one and the same physical quantity, a whole set of units of widely different sizes that are decimal multiples of each other.

The current version of the SI provides twenty metric prefixes that signify decimal powers ranging from 10^{−24} to 10^{24}. Apart from the prefixes for 1/100, 1/10, 10, and 100, all the other ones are powers of 1000.

Name | Prefix | Analogvalue | Digitalvalue |

Pico | p | 10^{-12} | – |

Nano | n | 10^{-9} | – |

Micro | µ | 10^{-6} | – |

Milli | m | 10^{-3} | – |

Kilo | k | 10^{3}(1000) | 2^{10}(1024) |

Mega | M | 10^{6}(1,000,000) | 2^{20}(1,048,576) |

Giga | G | 10^{9}(1,000,000,000) | 2^{30}(1,073,741,824) |

Tera | T | 10^{12}(1,000,000,000,000) | 2^{40}(1,099,511,627,776) |

#### Exemple:

mA | Milliamp = one thousandth of an ampere

km | Kilometre = one thousand metres

µv | Microvolt = one millionth of a volt

GW | Gigawatt = one thousand million watts

kW | Kilowatt = one thousand watts

#### Calculator example:

1 kilometre is 1 metre × 10^{3}

Enter into calculator 1 EXP 3 = (1000) metres

1000 metres is 1 kilometre × 10^{-3}

Enter into calculator 1000 EXP −3 = (1) kilometre

1 micro volt is 1 volt × 10^{-6}

Enter into calculator 1 EXP −6 = (1−06 or 0.000 001) volts

(note sixth decimal place).

**SI Defining Constants**

The SI rests on a foundation of seven values, known as the constants. The values of the constants are the same everywhere in the universe. In the SI, these constants completely define the seven base SI units:

Symbol | Name | Exact value |

hyperfine transition frequency of Cs | 9192631770 Hz | |

c | speed of light | 299792458 m/s |

h | Planck constant | 6.62607015×10^{−34} J⋅s |

e | elementary charge | 1.602176634×10^{−19} C |

k | Boltzmann constant | 1.380649×10^{−23} J/K |

N_{A} | Avogadro constant | 6.02214076×10^{23} mol^{−1} |

K_{cd} | luminous efficacy of 540 THz radiation | 683 lm/W |

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