Here are basic data associated with the International System of Units, commonly known as the "metric system". The abbreviation "SI" is from the French expression Système Internationale [d'Unités].
Several people come here really seeking a Metric Conversion Chart.
SI base units | |||
---|---|---|---|
quantity | name | symbol | definition |
length | metre | m | length of the path travelled by light in vacuum during a time interval of 1/299 792 458 second. |
mass | kilogram | kg | mass of international prototype. |
time | second | s | duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cæsium-133 atom. |
electric current | ampere | A | the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 metre apart in vacuum would produce between these conductors a force equal to 2 × 10^{-7} newton per square metre of length. |
thermodynamic temperature | kelvin | K | the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. |
amount of substance | mole | mol | the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. |
luminous intensity | candela | cd | the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10^{12} hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. |
SI derived units | |||
---|---|---|---|
quantity | name | symbol | definition |
plane angle | radian | rad | the plane angle between two radii of a circle that cut off on the circumference an arc equal in length to the radius |
solid angle | steradian | sr | solid angle that, having its vertex in the centre of a sphere, cuts off an area of the surface of the sphere equal to that of a square with sides of length equal to the radius of the sphere. |
frequency | hertz | Hz | 1 / s |
force | newton | N | m kg / s² |
pressure | pascal | Pa | N / m² |
energy | joule | J | N m |
power | watt | W | J / s |
electric charge | coulomb | C | A s |
electric potential | volt | V | W / A |
capacitance | farad | F | C / V |
resistance | ohm | Ω | V / A |
conductance | siemens | S | A / V |
magnetic flux | weber | Wb | V s |
magnetic flux density | tesla | T | Wb / m² |
inductance | henry | H | Wb / A |
luminous flux | lumen | lm | cd sr |
illuminance | lux | lx | lm / m² |
Units which may or may not be a part of the International System of Units (I just don't know) | |||
nuclear activity | becquerel | Bq | Hz |
radiation dose | sievert | Sv | J / kg |
absorbed dose | gray | Gy | Sv |
enzyme activity | katal | kat | mol / s |
Prefixes used with SI units | ||||||
---|---|---|---|---|---|---|
Factor | Prefix | Symbol | Factor | Prefix | Symbol | |
10^{1} | deca | da | 10^{-1} | deci | d | |
10^{2} | hecto | h | 10^{-2} | centi | c | |
10^{3} | kilo | k | 10^{-3} | milli | m | |
10^{6} | mega | M | 10^{-6} | micro | µ | |
10^{9} | giga | G | 10^{-9} | nano | n | |
10^{12} | tera | T | 10^{-12} | pico | p | |
10^{15} | peta | P | 10^{-15} | femto | f | |
10^{18} | exa | E | 10^{-18} | atto | a | |
10^{21} | zetta | Z | 10^{-21} | zepto | z | |
10^{24} | yotta | Y | 10^{-24} | yocto | y |
Prefixes are usually added onto the names of the base or derived units. The resultant word is best accented on the first syllable. For example, 1000 metres is 1 kilometre (1000 m = 1 km), pronounced KILL-a-me-ter / ˈkɪləˌmitɚ /.
But, with the kilogram, we pretend as though the base unit be the "gram" (1/1000 kg) for purposes of forming quantities of variant magnitudes: e.g., one millionth of a kilogram is not a microkilogram (µkg) but a milligram (mg).