SI
The International System of Units, abbreviated SI (from the original French Système International) is a global standard used to express the magnitudes or quantities of natural phenomena.
What is SI
SI stands for Système International, the name of the original metric system developed by the French. Today, it is used as an abbreviation for the International System of Units, the global standard of measurement. The vast majority of the countries around the world have officially adopted the SI. Since it is the modern form of the metric system, the SI is also referred to as the metric system. SI is the only system that has an official status in almost every country. Even in countries where it may not be used as a primary system of measurement, it is still widely used, particularly in contexts where measurements may be shared on a large or global scale such as international trade, manufacturing, health and safety, security, protection of the environment, and basic science.
The SI is a system comprised of base units and derived units.
- Base unit - In the SI, a base unit represents some physical quantity such as time, length, mass, and temperature such that none of the quantities can be expressed in terms of the other. For example, temperature cannot be expressed in terms of mass. There are 7 base units in the SI, and the SI uses prefixes that represent a decimal multiple or submultiple of the base unit it precedes. For example, the "kilo" in "kilometer" indicates 103 such that a kilometer is equal to 103 meters, or 1,000 meters.
- Derived unit - All other units in SI are referred to as derived units. These units are derived in terms of the base units of SI.
Base units
There are 7 SI base units:
| Name | Quantity |
|---|---|
| second (s) | time |
| metre (m) | length |
| kilogram (kg) | mass |
| ampere (A) | electric current |
| kelvin (K) | thermodynamic temperature |
| mole (mol) | amount of substance |
| candela (cd) | luminous intensity |
The precise definitions of the base units are as follows:
- second - the fixed numerical value of the caesium frequency ΔVcs, 9,192,631,770 when expressed in units of Hz, which is equal to s-1.
- meter - the length traveled by light in a vacuum in
of a second. - kilogram - the fixed numerical value of the Planck constant h (6.62607015 × 10-34) when expressed in units of J·s, which is equal to kg·m2·s-1.
- ampere - the fixed numerical value of the elementary charge (1.602176634 × 10-19) when expressed in coulombs, which is equal to A·s.
- kelvin - the fixed numerical value of the Boltzmann constant k (1.380649 × 10-23) when expressed in units of J·K-1, which is equal to kg·m2·s-2K-1.
- mole - the fixed numerical value of the Avogadro Constant (6.02214076 × 1023) when expressed in units of mol-1.
- candela - the candela is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation 540 × 1012 Hz, Kcd, to be 683 when expressed in units of lm·W-1, which is equal to cd·sr·W-1 or cd·sr·kg-1·m-2·s3.
Derived unit
SI also allows for an unlimited number of additional units to be derived from SI base units. These are referred to as SI derived units, which are units that can be represented as products of powers of the base units. There are many SI derived units which won't all be listed here. A few examples include degrees Celsius, watts, and volts.
On top of the many derived units of SI, there are also a number of non-SI units that are accepted for use with SI. Some of these include minutes, hours, days, liters, tonnes, and more. Below is a table of some of the named SI derived units.
| Name | Symbol | Quantity | SI base unit representation |
|---|---|---|---|
| radian | rad | plane angle |  |
| steradian | sr | solid angle |  |
| hertz | Hz | frequency |  |
| newton | N | force, weight |  |
| pascal | PA | pressure, stress |  |
| joule | J | energy, work, heat |  |
| watt | W | power, radiant flux |  |
| coulomb | C | electric charge |  |
| volt | V | electric potential, voltage, emf |  |
| farad | F | capacitance |  |
| ohm | Ω | resistance, impedance, reactance |  |
| siemens | S | electrical conductance |  |
| weber | Wb | magnetic flux |  |
| tesla | T | magnetic flux density |  |
| henry | H | inductance |  |
| degree Celsius | °C | temperature relative to 273.15 K |  |
| lumen | lm | luminous flux |  |
| lux | lx | illuminance |  |
| becquerel | Bq | activity referred to a radionuclide | |
| gray | Gy | absorbed dose of ionising radiation |  |
| sievert | Sv | equivalent dose of ionising radiation | |
| katal | kat | catalytic activity |  |
SI prefixes
SI prefixes are prefixes such as milli-, centi-, and kilo- that are added to an SI unit to indicate a multiple or submultiple of the unit. Each prefix indicates an integer power of 10. Kilo-, for example, indicates 103 or 1000, while milli- indicates 10-3 or 
.
A unit cannot have more than one SI prefix. For example, a "millikilogram" cannot exist. There are 24 specified SI prefixes.
| SI Prefixes table | |||
|---|---|---|---|
| Prefix | Symbol | Factor | Factor name |
| quetta | Q | 1030 | nonillion |
| ronna | R | 1027 | octillion |
| yotta | Y | 1024 | septillion |
| zetta | Z | 1021 | sextillion |
| exa | E | 1018 | quintillion |
| peta | P | 1015 | quadrillion |
| tera | T | 1012 | trillion |
| giga | G | 109 | billion |
| mega | M | 106 | million |
| kilo | k | 103 | thousand |
| hecto | h | 102 | hundred |
| deka | da | 101 | ten |
| 100 | one | ||
| deci | d | 10-1 | tenth |
| centi | c | 10-2 | hundredth |
| milli | m | 10-3 | thousandth |
| micro | μ | 10-6 | millionth |
| nano | n | 10-9 | billionth |
| pico | p | 10-12 | trillionth |
| femto | f | 10-15 | quadrillionth |
| atto | a | 10-18 | quintillionth |
| zepto | z | 10-21 | sextillionth |
| yocto | y | 10-24 | septillionth |
| ronto | r | 10-27 | octillionth |
| quecto | q | 10-30 | nonillionth |
100 is the base unit, with the exception of the kilogram. The kilogram is the only SI base unit that already has a prefix, and as mentioned, SI units cannot have more than one prefix. SI prefixes are applied to the unit of gram rather than the kilogram.