# SI Units

## SI Base units

SI (International System of Units) is an Internal Unit System based on 7 Units which are considered independent of Each Other.

Base quantity Name Symbol
length meter m
mass kilogram kg
time second s
electric current ampere A
thermodynamic temperature kelvin K
amount of substance mole mol
luminous intensity candela cd

## SI General Derived Units

SI derived units are calculated based on applying an equation on the SI Derived Units.

Table: SI General Derived Units
Quantity Name Symbol
area square meter m2
volume cubic meter m3
speed, velocity meter per second m/s
acceleration meter per second squared m/s2
wave number reciprocal meter m-1
mass density kilogram per cubic meter kg/m3
specific volume cubic meter per kilogram m3/kg
current density ampere per square meter A/m2
magnetic field strength ampere per meter A/m
amount-of-substance concentration mole per cubic meter mol/m3
luminance candela per square meter cd/m2
mass fraction kilogram per kilogram, which may be represented by the number 1 kg/kg = 1

## SI Derived Units with Special Names and Symbols

There are 22 SI Derived Units with Special Names and Symbols. These 22 Derived Units are combined to create a set of secondary derived SI Units, which will be shown in the table ‘Secondary Derived Units with Special Names and Symbols” down below.

Table: SI Derived Units with Special Names and Symbols
Quantity Name Symbol Expression using other Units Expression using SI base units
solid angle steradian (a) sr (c)   – m2·m-2 = 1 (b)
frequency hertz Hz   – s-1
force newton N   – m·kg·s-2
pressure, stress pascal Pa N/m2 m-1·kg·s-2
energy, work, quantity of heat joule J N·m m2·kg·s-2
power, radiant flux watt W J/s m2·kg·s-3
electric charge, quantity of electricity coulomb C   – s·A
electric potential difference,
electromotive force
volt V W/A m2·kg·s-3·A-1
electric resistance ohm V/A m2·kg·s-3·A-2
electric conductance siemens S A/V m-2·kg-1·s3·A2
magnetic flux weber Wb V·s m2·kg·s-2·A-1
magnetic flux density tesla T Wb/m2 kg·s-2·A-1
inductance henry H Wb/A m2·kg·s-2·A-2
Celsius temperature degree Celsius °C   – K
luminous flux lumen lm cd·sr (c) m2·m-2·cd = cd
illuminance lux lx lm/m2 m2·m-4·cd = m-2·cd
activity (of a radionuclide) becquerel Bq   – s-1
absorbed dose, specific energy (imparted), kerma gray Gy J/kg m2·s-2
dose equivalent (d) sievert Sv J/kg m2·s-2
catalytic activity katal kat s-1·mol

## SI Secondary Derived Units with Special Names and Symbols

SI Units mentioned in the following table are derived from other SI Derived Units with Special Names and Symbols, hence we can treat them as SI Secondary Derived Units.

Table: SI Secondary Drived Units with Special Names and Symbols
Quantity Unit Symbol
dynamic viscosity pascal second Pa·s
moment of force newton meter N·m
surface tension newton per meter N/m
heat flux density, irradiance watt per square meter W/m2
heat capacity, entropy joule per kelvin J/K
specific heat capacity, specific entropy joule per kilogram kelvin J/(kg·K)
specific energy joule per kilogram J/kg
thermal conductivity watt per meter kelvin W/(m·K)
energy density joule per cubic meter J/m3
electric field strength volt per meter V/m
electric charge density coulomb per cubic meter C/m3
electric flux density coulomb per square meter C/m2
exposure (x and rays) coulomb per kilogram C/kg