The International System of Units is a decimal system of weights and measures which is based on and extends the metric system. It is abbreviated as SI in all languages.
SI includes seven basic units (see table 1). The metre, defined to be 1,650,763.73 wavelengths in vacuum of the redorange line of the krypton86 spectrum, is the SI unit for length. The kilogram, which is approximately 2.2 pounds avoirdupois and equals 1,000 grams (as defined by the platinumiridium prototype kilogram held by the International Bureau of Weights and Measures in Sèvres, France), is the SI unit for mass. It is the sole base unit that remains defined by an artefact. It is also the only SI unit with a prefix as part of its name and symbol. The second, or the duration of 9,192,631,770 cycles of the radiation corresponding to a specified transition of the caesium133 atom, is the SI unit for time. The ampere is the SI unit for electric current. It is the constant current produced by one volt which, when maintained in two parallel conductors separated by one metre in vacuum, generates an electromagnetic force of 2 x 10^{7} N m^{1}. The kelvin, which is equal to 1/273.16 of the thermodynamic temperature at the triple point of water, is the SI unit for thermodynamic temperature. The kelvin’s magnitude equals that of the degree Celsius; however, a temperature expressed in degrees Celsius is the numerical equivalent of the temperature in kelvins less 273.15. The mole is the SI unit for amount of substance; it contains as many elementary units of substance as there are atoms in 0.012 kg of carbon12. Elementary units must be specified, as they may be atoms, electrons, ions, molecules, radicals, etc. The candela is the SI unit for luminous intensity. It equals the luminous intensity of the blackbody radiation, in the perpendicular direction, from an area of 1/600,000 square metres at platinum’s freezing temperature (2,042 kelvins) under 101,325 pascals of pressure, which approximates the intensity of a single paraffin candle.
Table 1. SI base units
Quantity 
SI unit name 
Symbol 
Length 
Metre 
m 
Mass^{1} 
Kilogram 
kg 
Time 
Second 
s 
Electric current 
Ampere 
A 
Thermodynamic temperature 
Kelvin^{2} 
K 
Amount of substance 
Mole 
mol 
Luminous intensity 
Candela 
cd 
^{1} "Weight” is often used to mean “mass”.
^{2} The name “degree kelvin” and the symbol “degK” were declared obsolete
at a 1967 international conference.
SI also includes two supplementary units (see table 2). Radian and steradian are the dimensionless units for the dimensionless quantities plane angle and solid angle, respectively. Units for other quantities are derived from the seven basic and two supplementary units.
Table 2. SI supplementary units
Quantity 
SI unit name 
Symbol 
Expression in terms of SI base units 
Plane angle 
Radian 
rad 
m·m^{–} ^{1} =1 
Solid angle 
Steradian 
sr 
m^{2} ·m^{–} ^{ } ^{2} =1 
Table 3 lists selected SI derived units expressed in terms of base units. Derived units with special names and symbols are listed in table 4. These may be used to express other derived units (see table 5). The two supplementary units may also be used to express derived units (see table 6).
The 16 prefixes used to create multiples and submultiples of SI units are listed in table 7. Since multiple prefixes cannot be used, these prefixes are used with gram (g), but not with kilogram (kg).
A number of units that are not part of SI are widely used, especially in the United States. Those which are considered acceptable for use with SI in the US are listed in table 8. A conversion table for SI units is provided in table 9.
Table 3. Selected SI derived units expressed in terms of base units
Quantity 
SI unit name 
Symbol 
Area 
Square metre 
m^{2} 
Volume 
Cubic metre 
m^{3} 
Speed, velocity 
Metre per second 
m/s 
Acceleration 
Metre per second squared 
m/s^{2} 
Wave number 
Reciprocal metre 
m^{–} ^{1} 
Density, mass density 
Kilogram per cubic metre 
kg/m^{3} 
Specific volume 
Cubic metre per kilogram 
m^{3}/kg 
Current density 
Ampere per square metre 
A/m^{2} 
Magnetic field strength 
Ampere per metre 
A/m 
Concentration (of amount of substance) 
mole per cubic metre 
mol/m^{3} 
Luminance 
Candela per square metre 
cd/m^{2} 
Table 4. SI derived units with special names
Quantity 
SI unit name 
Symbol 
Expression in terms 
Frequency 
Hertz 
Hz 
s^{–} ^{1} 
Force 
Newton 
N 
m·kg/s^{2} 
Pressure, stress 
Pascal 
Pa 
N/m^{2} 
Energy, work, quantity of heat 
Joule 
J 
N·m 
Power, radiant flux 
Watt 
W 
J/s 
Electric charge, quantity of electricity 
Coulumb 
C 
s·A 
Electric potential, potential difference, electromotive force 
Volt 
V 
W/A 
Capacitance 
Farad 
F 
C/V 
Electric resistance 
Ohm 
Omega 
V/A 
Electric conductance 
Siemens 
S 
A/V 
Magnetic flux 
Weber 
Wb 
V·s 
Magnetic flux density 
Tesla 
T 
Wb/m^{2} 
Inductance 
Henry 
H 
Wb/A 
Celsius temperature^{1} 
Degree Celsius 
C 
K 
Luminous flux 
Lumen 
lm 
cd·sr 
Activity (of a radionuclide) 
Becquerel 
Bq 
s^{–} ^{1} 
Absorbed dose, specific energy imparted, kerma, absorbed dose index 
Gray 
Gy 
J/kg 
Dose equivalent, dose equivalent index 
Sievert 
Sv 
J/kg 
^{1} In addition to the thermodynamic temperature (T) expressed in kelvins (see table 105.1), Celsius
temperature (t) is also used and is defined by the equation t = T – T_{0} where T_{0} = 273.15 K by
definition. The unit “degree Celsius”, which is equal to the unit “kelvin”, is used to express Celsius
temperature. Here, the term “degree Celsius” is a special name substituted for “kelvin”.
However, a difference or interval of Celsius temperature can be expressed in either kelvins
or degrees Celsius.
Table 5. Examples of SI derived units expressed with special names
Quantity 
SI unit name 
Symbol 
Dynamic viscosity 
Pascal second 
Pa·s 
Moment of force 
Newton metre 
N·m 
Surface tension 
Newton per metre 
N/m 
Heat flux density, irradiance 
Watt per square metre 
W/m^{2} 
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 metre kelvin 
W/(m·K) 
Energy density 
Joule per cubic metre 
J/m^{3} 
Electric field strength 
Volt per metre 
V/m 
Electric charge density 
Coulomb per cubic metre 
C/m^{3} 
Electric flux density 
Coulomb per square metre 
C/m^{2} 
Permittivity 
Farad per metre 
F/m 
Permeability 
Henry per metre 
H/m 
Molar energy 
Joule per metre 
J/mol 
Molar entropy, molar heat capacity 
Joule per mole kelvin 
J/(mol·K) 
Exposure (x and gamma rays) 
Coulomb per kilogram 
C/kg 
Absorbed dose rate 
Gray per second 
Gy/s 
Table 6. Examples of SI derived units formed with supplementary units
Quantity 
SI unit name 
Symbol 
Angular velocity 
Radian per second 
rad/s 
Angular acceleration 
Radian per second squared 
rad/s^{2} 
Radiant intensity 
Watt per steradian 
W/sr 
Radiance 
Watt per square metre steradian 
W/(m^{2}·sr) 
Table 7. SI prefixes
Factor 
Prefix 
Symbol 
10^{18}^{ } 
exa 
E 
10^{15} 
peta 
P 
10^{12} 
tera 
T 
10^{9} 
giga 
G 
10^{6} 
mega 
M 
10^{3} 
kilo 
k 
10^{2} 
hecto 
h 
10^{1} 
deka 
da 
10^{} ^{1} 
deci 
d 
10^{} ^{2} 
centi 
c 
10^{} ^{3} 
milli 
m 
10^{} ^{6} 
micro 
μ 
10^{} ^{9} 
nano 
n 
10^{} ^{12} 
pico 
p 
10^{} ^{15} 
femto 
f 
10^{} ^{18} 
atto 
a 
Table 8. Units in use with SI
Name 
Symbol 
Value in SI unit 
Minute (time) 
min 
1 min = 60 s 
Hour 
h 
1 h = 60 min = 3,600 s 
Day 
d 
1 d = 24 h = 86,400 s 
Degree (angle) 

1 = (pi/180) rad 
Minute (angle) 

1 = (1/60) = (pi/10,800) rad 
Second (angle) 

1 = (1/60) = (pi/648,000) rad 
Litre 
l^{1} 
1 l = 1 dm^{3} = 10^{–} ^{3} m^{3} 
Tonne^{2} 
t 
1 t = 10^{3} kg 
Hectare (land area) 
ha 
1 ha = 1 hm^{2} = 10^{4} m^{2 } 
Electronvolt^{3} 
eV 
1 eV = 1.602 18 x 10^{–} ^{19} J 
Unified atomic mass unit^{3} 
u 
1 u = 1.660 54 x 10^{–} ^{27} kg 
^{1} Both “l” and “L” are accepted as symbols for litre.
^{2} In some countries, such as the United States, “metric ton” is used instead of “tonne”.
^{3} The values of these units in SI units are not known exactly; the values must be obtained
through experiment. The electronvolt is the kinetic energy acquired by an electron passing
through a potential difference of 1 volt in a vacuum. The unified atomic mass unit equals 1/12 of
the mass of the atom of the nuclide ^{12}C.
Table 9. Conversion of nonSI units to SI units
From/to 
To/from 
Multiply by/divide by 
Inch (in) 
m 
2.54 x 10^{–} ^{2} 
Feet (ft) 
m 
0.3048 
Square inch (in^{2} ) 
m^{2} 
6.4516 x 10^{–} ^{4} 
Square foot (ft^{2} ) 
m^{2} 
9.2903 x 10^{–} ^{2} 
Cubic inch (in^{3} ) 
m^{3} 
1.638 71 x 10^{–} ^{5} 
Cubic foot (ft^{3} ) 
m^{3} 
2.831 68 x 10^{–} ^{2} 
Litre (l) 
m^{3} 
10^{–} ^{3} 
Gallon (gal) 
m^{3} 
4.546 09 x 10^{–} ^{3} 
Mile/hour (mi hr^{–} ^{1} ) 
m s^{–} ^{1} 
0.477 04 
Kilometre/hour (km hr^{–} ^{1} ) 
m s^{–} ^{1} 
0.277 78 
Pound (lb) 
kg 
0.453 592 
Gram/cm^{3} (g cm^{–} ^{3} ) 
kg m^{–} ^{3} 
10^{3} 
Pound/in^{3} 
kg m^{–} ^{3} 
2.767 99 x 10^{4} 
mmHG 
Pa 
133.322 
Atmosphere (atm) 
Pa 
1.013 25 x 10^{5} 
Horsepower (hp) 
W 
745.7 
erg 
J 
10^{–} ^{7} 
Electronvolt (eV) 
J 
1.602 10 x 10^{–} ^{19} 
Kilowatthour (kW hr) 
J 
3.6 x 10^{6} 
Calorie (cal) 
J 
4.1868 
Dyne 
N 
10^{–} ^{5} 
kgf 
N 
9.806 65 
Poundal 
N 
0.138 255 
lbf 
N 
4.448 22 
Acknowledgement: The information in the tables is based primarily on data from the US National Institute of Standards and Technology (NIST).