# The International System of Units (SI)Cheat Sheet

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 × 1012 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
101 deca da 10-1 deci d
102 hecto h 10-2 centi c
103 kilo k 10-3 milli m
106 mega M 10-6 micro µ
109 giga G 10-9 nano n
1012 tera T 10-12 pico p
1015 peta P 10-15 femto f
1018 exa E 10-18 atto a
1021 zetta Z 10-21 zepto z
1024 yotta Y 10-24 yocto y

## Prefix Notes

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).