What is the International System of Units? | General Chemistry 1

What is the International System of Units (SI)?

The International System of Units (abbreviated SI from French, but also known as the metric system) defines a set of units for measuring physical quantities and their multiples. It consists of seven defining constants, seven base units, and 22 derived units. 


The 7 Defining Constants

The 7 Base Units

  • Length - meter (m)
  • Time - second (s)
  • Amount of substance - mole (mole)
  • Electric current - ampere (A)
  • Temperature - kelvin (K)
  • Luminous intensity - candela (cd)
  • Mass - kilogram (kg)


The 22 Derived Units

Name Symbol Quantity Equivalents SI base unit Equivalents
hertz  Hz frequency  1/s  s−1
radian rad  angle  m/m     1
steradian sr  solid angle  m2/m 1
newton  N force, weight  kg⋅m/s2  kg⋅m⋅s−2
pascal  Pa  pressure, stress  N/m kg⋅m−1⋅s−2
joule energy, work, heat m⋅N, C⋅V, W⋅s   kg⋅m2⋅s−2
watt  W power, radiant flux   J/s, V⋅A   kg⋅m2⋅s−3
coulomb C electric charge or quantity of electricity  s⋅A, F⋅V s⋅A
volt V voltage, electrical potential difference, electromotive force    W/A, J/C kg⋅m2⋅s−3⋅A−1
farad   electrical capacitance  C/V, s/Ω  kg−1⋅m−2⋅s4⋅A2
ohm Ω electrical resistance, impedance, reactance   1/S, V/A  kg⋅m2⋅s−3⋅A−2
siemens S electrical conductance 1/Ω, A/V  kg−1⋅m−2⋅s3⋅A2
weber  Wb  magnetic flux J/A, T⋅m2,V⋅s  kg⋅m2⋅s−2⋅A−1
tesla  T magnetic induction, magnetic flux density   V⋅s/m2, Wb/m2, N/(A⋅m)  kg⋅s−2⋅A−1
henry H electrical inductance  V⋅s/A, Ω⋅s, Wb/A  kg⋅m2⋅s−2⋅A−2
degree Celsius °C   temperature relative to 273.15 K  K K
lumen lm  luminous flux  cd⋅sr  cd
lux lx illuminance lm/m2  cd⋅m−2
becquerel  Bq  radioactivity (decays per unit time)  1/s s−1
gray   Gy  absorbed dose (of ionizing radiation)   J/kg  m2⋅s−2
sievert  Sv   equivalent dose (of ionizing radiation) J/kg  m2⋅s−2
katal kat  catalytic activity mol/s  s−1⋅mol



What are Examples of Derived Units?

Examples of Kinematic SI Derived Units

Name Symbol Quantity Expression in terms of SI base units
metre per second m/s  speed, velocity m⋅s−1
metre per second squared m/s2  acceleration  m⋅s−2
metre per second cubed m/s3 jerk, jolt m⋅s−3
metre per second to the fourth m/s4  snap, jounce  m⋅s−4
radian per second rad/s angular velocity  s−1
radian per second squared rad/s2  angular acceleration s−2
hertz per second Hz/s  frequency drift  s−2
cubic metre per second  m3/s volumetric flow m3⋅s−1


Examples of Mechanical SI Derived Units

Name Symbol Quantity Expression in terms of SI base units
square metre m2 area m2
cubic metre m3 volume m3
newton N force m⋅kg⋅s−2
newton-second N⋅s momentum, impulse m⋅kg⋅s−1
newton metre second N⋅m⋅s    angular momentum  m2⋅kg⋅s−1
newton-metre N⋅m = J/rad torque, moment of force m2⋅kg⋅s−2
newton per second N/s yank m⋅kg⋅s−3
reciprocal metre m−1 wavenumber, optical power, curvature, spatial frequency  m−1
kilogram per square metre kg/m2 area density m−2⋅kg
kilogram per cubic metre kg/m3 density, mass density m−3⋅kg
cubic metre per kilogram m3/kg specific volume m3⋅kg−1
joule-second             J⋅s action m2⋅kg⋅s−1
joule per kilogram             J/kg specific energy m2⋅s−2
joule per cubic metre            J/m3  energy density m−1⋅kg⋅s−2
newton per metre N/m = J/m2 surface tension, stiffness kg⋅s−2
watt per square metre             W/m2 heat flux density, irradiance kg⋅s−3
square metre per second            m2/s  kinematic viscosity, thermal diffusivity, diffusion coefficient m2⋅s−1
pascal-second          Pa⋅s = N⋅s/m dynamic viscosity  m−1⋅kg⋅s−1
pressure           Force per area  m−1⋅kg⋅s−2
kilogram per metre             kg/m linear mass density m−1⋅kg
kilogram per second            kg/s mass flow rate  kg⋅s−1
watt per steradian square metre           W/(sr⋅m2) radiance  kg⋅s−3
watt per steradian cubic metre             W/(sr⋅m3) spect radiance m−1⋅kg⋅s−3
watt per metre            W/m  spectral power m⋅kg⋅s−3
gray per second            Gy/s  absorbed dose rate m2⋅s−3
metre per cubic metre         m/m3   fuel efficiency m−2
watt per cubic metre            W/m3 spectral irradiance, power density  m−1⋅kg⋅s−3
joule per square metre second            J/(m2⋅s) energy flux density  kg⋅s−3
reciprocal pascal             Pa−1 compressibility m⋅kg−1⋅s2
joule per square metre            J/m2 radiant exposure  kg⋅s−2
kilogram square metre          kg⋅m2 moment of inertia m2⋅kg
newton metre second per kilogram          N⋅m⋅s/kg  specific angular momentum   m2⋅s−1
watt per steradian           W/sr radiant intensity   m2⋅kg⋅s−3
watt per steradian metre         W/(sr⋅m)   spectral intensity m⋅kg⋅s−3


Examples of Molar SI Derived Units

Name Symbol Quantity Expression in terms
of SI base units
mole per cubic metre  mol/m3 molarity, amount of substance concentration  m−3⋅mol
cubic metre per mole             m3/mol molar volume m3⋅mol−1
joule per kelvin mole            J/(K⋅mol)  molar heat capacity, molar entropy m2⋅kg⋅s−2⋅K−1⋅mol−1
joule per mole            J/mol molar energy  m2⋅kg⋅s−2⋅mol−1
siemens square metre per mole           S⋅m2/mol molar conductivity   kg−1⋅s3⋅A2⋅mol−1
mole per kilogram            mol/kg  molality kg−1⋅mol
kilogram per mole             kg/mol molar mass kg⋅mol−1
cubic metre per mole second          m3/(mol⋅s)   catalytic efficiency  m3⋅s−1⋅mol−1
reciprocal mole          mol−1  Avogadro constant   mol−1


Examples of Electromagnetic SI Derived Units

Name Symbol Quantity Expression in terms
of SI base units
coulomb per square metre C/m2 electric displacement field, polarization density m−2⋅s⋅A
coulomb per cubic metre C/m3 electric charge density m−3⋅s⋅A
ampere per square metre             A/m2 electric current density m−2⋅A
siemens per metre    S/m  electrical conductivity  m−3⋅kg−1⋅s3⋅A2
farad per metre         F/m   permittivity m−3⋅kg−1⋅s4⋅A2
henry per metre            H/m  magnetic permeability m⋅kg⋅s−2⋅A−2
volt per metre            V/m electric field strength  m⋅kg⋅s−3⋅A−1
ampere per metre             A/m magnetization, magnetic field strength m−1⋅A
coulomb per kilogram          C/kg exposure (X and gamma rays)  kg−1⋅s⋅A
ohm metre          Ω⋅m  resistivity m3⋅kg⋅s−3⋅A−2
coulomb per metre             C/m linear charge density m−1⋅s⋅A
joule per tesla             J/T magnetic dipole moment m2⋅A
square metre per volt second           m2/(V⋅s) electron mobility kg−1⋅s2⋅A
reciprocal henry          H−1  magnetic reluctance m−2⋅kg−1⋅s2⋅A2
weber per metre            Wb/m magnetic vector potential  m⋅kg⋅s−2⋅A−1
weber metre             Wb⋅m magnetic moment m3⋅kg⋅s−2⋅A−1
tesla metre             T⋅m magnetic rigidity m⋅kg⋅s−2⋅A−1
ampere radian            A⋅rad  magnetomotive force A
metre per henry            m/H  magnetic susceptibility m−1⋅kg−1⋅s2⋅A2


Examples of Photometric SI Derived Units

Name Symbol Quantity Expression in terms
of SI base units
lumen second            lm⋅s  luminous energy s⋅cd
lux second            lx⋅s luminous exposure  m−2⋅s⋅cd
candela per square metre            cd/m2 luminance  m−2⋅cd
lumen per watt            lm/W  luminous efficacy m−2⋅kg−1⋅s3⋅cd


Examples of Thermodynamic SI Derived Units

Name Symbol Quantity Expression in terms
of SI base units
joule per kelvin             J/K heat capacity, entropy m2⋅kg⋅s−2⋅K−1
joule per kilogram kelvin          J/(K⋅kg)  specific heat capacity, specific entropy   m2⋅s−2⋅K−1
watt per metre kelvin            W/(m⋅K) thermal conductivity  m⋅kg⋅s−3⋅K−1
kelvin per watt             K/W thermal resistance m−2⋅kg−1⋅s3⋅K
reciprocal kelvin             K−1 thermal expansion coefficient K−1
kelvin per metre            K/m temperature gradient  m−1⋅K


What is an Example of a Non-SI Unit?


Units Officially Accepted for Use With the SI

Name Symbol Quantity Value in SI units
minute min time 1 min = 60 s
hour h time 1 h = 60 min = 3 600 s
day d time 1 d = 24 h = 1440 min = 86 400 s
astronomical unit au length 1 au = 149 597 870 700 m
degree ° plane angle 1° = (π/180) rad
minute plane angle 1′ = (1/60)° = (π/10 800) rad
second plane angle 1″ = (1/60)′ = (1/3 600)° = (π/648 000) rad
hectare ha area 1 ha = 1 hm2 = 10 000 m2
litre l volume 1 l = 1 dm3 = 1 000 cm3 = 0.001 m3
tonne t mass 1 t = 103 kg
dalton Da mass 1 Da = 1.66053906660(50)×10−27 kg = 1.660 539 066 60(50) yg
electronvolt eV energy 1 eV = 1.602176634×10−19 J = 160.217 663 4 zJ
neper Np logarithmic ratio quantity
bel, decibel B, dB logarithmic ratio quantity


Units Not Officially Accepted for Use With the SI

Name Symbol Quantity Equivalent SI unit
gal Gal acceleration 1 Gal = 1 cm⋅s−2 = 0.01 m⋅s−2
unified atomic mass unit u mass 1 u = 1 Da = 1.66053906660(50)×10−27 kg
volt-ampere reactive var reactive power 1 var = 1 V⋅A

International System of Quantities

Base Quantities

Base quantity Symbol for quantity Symbol for dimension SI base unit SI unit symbol
length L metre m
mass m M kilogram kg
time t T second s
electric current I I ampere A
thermodynamic temperature T θ kelvin K
amount of substance n N mole mol
luminous intensity Iv J candela cd


Derived Quantities

Derived quantity Expression in SI base dimensions
plane angle 1
solid angle 1
frequency T-1
force LMT-2
pressure L-1MT-2
velocity LT-1
area L2
volume L3
acceleration LT-2



What is a Derived Quantity in Chemistry?

Derived quantities are not directly measurable, but they can be derived from measurements made. Instead, it's derived from two or more measurements and may include things such as area or volume calculation in physical sciences. Additionally, derived quantities can only be computed.


What Units are Commonly Used in Chemistry?

There are typically five SI base units chemists commonly use which are:

  1. the Kelvin (temperature)
  2. the kilogram (weight)
  3. the mole (amount)
  4. the meter (length)
  5. the second (time)


What is the International System of Units Based On?

The International System of Units (SI) is based on the metric system. The metric system is the most commonly used measurement system in the world. Only three countries - Liberia and Myanmar (with a combined population exceeding 60 million), and the USA don't use it. Every other country uses this form of weights & measures.


Who Created the SI?

According to the NIST, the International System of Units, or SI for short (from the French Le Système international d'unités), is a modern metric system that was established in 1960 by the 11th General Conference on Weights and Measures Conférence Générale des Poids et Mesures). This conference mandated all countries around the world to use these uniform units so we could have one set standard across every country.


Why Do We Need an International System?

The System Internationale (SI) is an important unit of measurement that can be used in all sorts of endeavors. For example, it's often the case for scientists and engineers who must make calculations involving both length and time to be able to work with equations and formulas that are universally translatable. It's also the most commonly used units system for international trade, so it not only supports scientific and technological research but also helps governments keep their economies running smoothly.