The International System of Units is abbreviated “SI” from the French version of the name. It is the modern metric system of measurement and it’s the most widely used system in everyday commerce and in science. The SI is maintained by a small agency in Paris, the International Bureau of Weights and Measures. The older metric system included several groups of units and was formerly called the meter-kilogram-second (MKS) system. The key features of the International System are a system of prefixes, decimalization, and a standard defined in terms of an invariable physical measure.
Base Units
The International System has seven base units from which all other units in the system are derived.
| Unit | Quantity | Symbol |
|---|---|---|
| meter | length | m |
| kilogram | mass | kg |
| second | time | s |
| ampere | electric current | A |
| kelvin | temperature | K |
| mole | amount of matter | mol |
| candela | luminous intensity | cd |
Supplementary Dimensionless Units
The International System uses two supplementary dimensionless units that are based on abstract geometrical concepts rather than physical standards. They consist of the radian and steradian.
| Unit | Quantity | Symbol |
|---|---|---|
| radian | plane angles | rad |
| steradian | solid angles | sr |
Prefixes
A multiple of a unit in the International System is formed by adding a prefix to the name of that unit. The prefixes are used to change the magnitude of the unit by orders of ten from 1024 to 10-24
| Prefix | Symbol | Multiplying Factor |
|---|---|---|
| yotta- | Y | 1024 = 1,000,000,000,000,000,000,000,000 |
| zetta- | Z | 1021 = 1,000,000,000,000,000,000,000 |
| exa- | E | 1018 = 1,000,000,000,000,000,000 |
| peta- | P | 1015 = 1,000,000,000,000,000 |
| tera- | T | 1012 = 1,000,000,000,000 |
| giga- | G | 109 = 1,000,000,000 |
| mega- | M | 106 = 1,000,000 |
| kilo- | K | 103 = 1,000 |
| hecto- | h | 102 = 1000 |
| deca- | da | 10 = 10 |
| deci- | d | 10-1 = 0.1 |
| centi- | c | 10-2 = 0.01 |
| milli- | m | 10-3 = 0.001 |
| micro- | µ | 10-6 = 0.000,001 |
| nano- | n | 10-9 = 0.000,000,001 |
| pico- | p | 10-12 = 0.000,000,000,001 |
| femto- | f | 10-15 = 0.000,000,000,000,001 |
| atto- | a | 10-18 = 0.000,000,000,000,000,001 |
| zepto- | z | 10-21 = 0.000,000,000,000,000,000,001 |
| yocto- | y | 10-24 = 0.000,000,000,000,000,000,000,001 |
Additional Units
Other units outside the International System that are currently accepted, subject to further review.
| Unit | Quantity | Symbol |
|---|---|---|
| angstrom (= 10-10m) | length | Å |
| electron-volt (= 0.160 aJ) | energy | eV |
| hectare (= 10,000 m2) | land area | ha |
| liter (= 1.0dm3) | volume or capacity | l |
| standard atmosphere (= 101.3 kPa) | pressure | atm |
Derived Units
Most of the units in the International System are actually derived units. In the below tables are examples of derived units containing names and symbols and those without names and symbols.
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