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The Metric SystemA reference to the world's most widely used system of measurement[The Metric System] [Values of Constants] [Conversion Factors] 

Unit Type  Unit Name  Symbol  Definition 

Length  metre  m  1/299,792,458 of the distance travelled by light in a vacuum in one second 
Mass  kilogram  kg  The mass of a PlatinumIridium cylinder kept in Paris. 
Time  second  s  The duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the Caesium 133 atom. 
Electric Current 
ampere  A  The amount of current that if maintained in two parallel rectilinear conductors (of infinite length and negligible circular crosssection) placed 1 metre apart in a vacuum, would produce a force between these conductors equal to 2 × 10^{7} newton per metre. 
Temperature  kelvin  K  The fraction 1/273.16 of the thermodynamic temperature at the triple point of water. 
Amount of Substance 
mole  mol  The amount of substance which contains as many elementary units as there are atoms in 12 grams of pure Carbon 12. 
Luminous Intensity 
candela  cd  The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^{12} hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. 
Angle  radian  rad  The angle subtended at the centre of a circle by an arc of the circumference equal in length to the radius of the circle. 
Solid Angle  steradian  sr  The solid angle subtended at the centre of a sphere of radius r by a portion of the surface of the sphere having an area r^{2}. 
For example, kilo means 1000 and can be used to denote 1000 of any unit (eg. 1000 metres is a kilometre, written as km). Also, milli means 1/1000th and can be used to denote 1/1000th of any unit (1/1000th of a metre is a millimetre, written as mm).
Note that the case of the prefix is important. M is mega, while m is milli.
Prefix Name  

tera  T  10^{12} 
giga  G  10^{9} 
mega  M  10^{6} 
kilo  k  10^{3} 
centi  c  10^{2} 
milli  m  10^{3} 
micro  µ  10^{6} 
nano  n  10^{9} 
pico  p  10^{12} 
femto  f  10^{15} 
atto  a  10^{18} 
The Fundamental Units are shown in red.
Note that the case of the unit is important. K is a degree Kelvin, k is kilo (1000).
Unit Type  Unit Name  Fundamental Units  

Length  metre  m  
Area  square metre  m^{2}  
Volume  cubic metre  m^{3}  
Time  second  s  
Frequency  hertz  Hz  s^{1} 
Velocity  metre per second  m s^{1}  
Acceleration  metre per second squared  m s^{2}  
Kinematic Viscosity  square metre per second  m^{2} s^{1}  
Mass  kilogram  kg  
Density  kilogram per cubic meter  kg m^{3}  
Moment of Inertia  kilogram square meter  kg m^{2}  
Force / Weight  newton  N  kg m s^{2} 
Momentum / Impulse  newton second  N s  kg m s^{1} 
Energy / Work / Torque  joule  J  kg m^{2} s^{2} (N m) 
Power  watt  W  kg m^{2} s^{3} (J s^{1}) 
Sound Intensity  watt per square meter  W m^{2}  
Surface Tension  newton per meter  N m^{1}  
Pressure  pascal  Pa  N m^{2} 
Compressibility  square meter per newton  m^{2} N^{1}  
Dynamic Viscosity  newton second per square meter  N s m^{2}  
Temperature  kelvin  K  
Thermal Conductivity  watt per metre per kelvin  W m^{1} K^{1}  
Entropy / Heat Capacity  joule per kelvin  J K^{1}  
Specific Heat Capacity  joule per kilogram per kelvin  J kg^{1} K^{1}  
Amount of Substance  mole  mol  
Ionic Strength  mole per kilogram  mol kg^{1}  
Molar Volume  cubic metre per mole  m^{3} mol^{1}  
Molar Mass  kilogram per mole  kg mol^{1}  
Luminous Intensity  candela  cd  
Luminance  candela per square metre  cd m^{2}  
Solid Angle  steradian  sr  
Luminous Flux  lumen  lm  cd sr 
Illumination  lux  lx  cd sr m^{2} 
Electric Current  ampere  A  
Magnetic Field Strength  ampere per metre  A m^{1}  
Electric Current Density  ampere per square metre  A m^{2}  
Magnetic Moment  ampere square metre  A m^{2}  
Electric Charge  coulomb  C  A s 
Dipole Moment  coulomb meter  C m  
Surface Charge Density  coulomb per square meter  C m^{2}  
Volume Charge Density  coulomb per cubic meter  C m^{3}  
Potential Difference  volt  V  kg m^{2} s^{3} A^{1} 
Electric Field Strength  volt per meter  V m^{1}  
Thermoelectric Power  volt per kelvin  V K^{1}  
Magnetic Flux  weber  Wb  V s 
Inductance  henry  H  Wb A^{1} 
Permeability  henry per meter  H m^{1}  
Flux Density  tesla  T  kg s^{2} A^{1} 
Resistance / Impedance  ohm  W  V A^{1} 
Resistivity  ohm meter  V A^{1} m  
Capacitance  farad  F  A^{2} s^{4} kg^{1} m^{2} 
Permittivity  farad per meter  F m^{1}  
Conductance  siemens  S  F s^{1} 
Electrical Conductivity  siemens per meter  S m^{1}  
Molar Conductivity  siemens square meter per mole  S m^{2} mol^{1}  
Angle  radian  rad  
Power of Lens  radian per meter  rad m^{1}  
Angular Velocity  radian per second  rad s^{1}  
Angular Acceleration  radian per second squared  rad s^{2} 
Quantity  Value  

Velocity of light in a vacuum  c  2.997 925 × 10^{8} m s^{1} 
Boltzmann Constant  k  1.380 54 × 10^{23} J K^{1} 
Planck Constant  h  6.625 59 × 10^{34} J s 
Gravitational Constant  G  6.673 × 10^{11} N m^{2} kg^{2} 
Acceleration of gravity on the Earth's surface 
g  9.806 65 m s^{2} 
Stefan Constant  s  5.670 3 × 10^{8}J K^{4} m^{2} s^{1} 
Faraday Constant  F  9.648 70 × 10^{4} C mol^{1} 
Avogadro Constant  L  6.022 52 × 10^{23} mol^{1} 
Charge on an electron  e  1.602 10 × 10^{19} C 
Rest mass of electron  m_{e}  9.109 08 × 10^{31} kg 
Hubble Constant  H  2.5 × 10^{18} m s^{1} m^{1} 
Gas Constant  R  8.314 34 J K^{1} mol^{1} 
Note that many of these units differ by country. This makes them of little use in a global environment, like the WORLD Wide Web or transnational business.
Unit  Description  Value 

Ångström  Length (Atomic)  1.000 × 10^{10} m 
acre  Area of Land  4.047 × 10^{3} m^{2} 
astronomical unit  Distance Between Earth and Sun  1.496 × 10^{11} m 
atmosphere  Atmospheric Pressure on the surface of the Earth at Sea Level 
1.013 × 10^{5} N m^{2} 
atomic mass unit  Mass of Hydrogen Atom  1.660 × 10^{27} kg 
barrel  Volume of Oil  0.1636 m^{3} 
British thermal unit  Energy  1.055 × 10^{3} J 
bushel (UK)  Volume (Dry Substances)  3.637 × 10^{2} m^{3} 
bushel (USA)  Volume (Dry Substances)  3.531 × 10^{2} m^{3} 
calorie  Energy  4.184 J 
carat  Weight  2 × 10^{4} kg 
chain  Length  20.12 m 
cubic foot  Volume  2.832 × 10^{2} m^{3} 
cubic yard  Volume  0.7646 m^{3} 
curie  Radioactivity  3.7 × 10^{10} s^{1} 
day  Time  8.6400 × 10^{4} s 
degree  Angle  1.745 × 10^{2} rad 
degree Celsius  Temperature  1.000 K 
degree Fahrenheit  Temperature  0.5556 K 
dyne  Force  1.000 × 10^{5} N 
electron volt  Energy (Atoms and SubAtomic Particles)  1.602 × 10^{19} J 
erg  Energy  1.000 × 10^{7} J 
fathom  Distance (Depth)  1.8288 m 
fluid ounce (UK)  Volume (Liquid)  2.841 × 10^{5} m^{3} 
foot  Length  0.3048 m 
foot pound force  Energy  1.356 × 10^{5} J 
gallon (UK)  Volume (Liquid)  4.546 × 10^{3} m^{3} 
gallon (USA)  Volume (Liquid)  3.785 × 10^{3} m^{3} 
gill (UK)  Volume (Liquid)  1.4205 × 10^{4} m^{3} 
gill (USA)  Volume (Liquid)  2.841 × 10^{4} m^{3} 
hectare  Area of Land  1.000 × 10^{4} m^{2} 
hour  Time  3.600 × 10^{3} s 
horsepower  Power  7.457 × 10^{2} W 
hundredweight (UK)  Weight  50.80 kg 
inch  Length  2.54 × 10^{2} m 
kilowatt hour  Energy  3.600 × 10^{6} J 
knot  Velocity (at Sea / Flight)  0.5114 m s^{1} 
league  Distance  9.0885 km 
light year  Distance (Stars / Galaxies)  9.461 × 10^{12} km 
litre  Volume (Liquid)  1.000 × 10^{3} m^{3} 
mile  Distance  1.609 km 
mile (nautical)  Distance (Sea / Air)  1.852 km 
mile per hour  Velocity  0.4704 m s^{1} 
minute  Angle  2.909 × 10^{4} rad 
minute  Time  60 s 
ounce  Weight  2.835 × 10^{2} kg 
parsec  Distance (Stars / Galaxies)  3.084 × 10^{13} km 
peck (UK)  Volume (Dry)  9.092 × 10^{3} m^{3} 
peck (USA)  Volume (Dry)  8.810 × 10^{3} m^{3} 
pint (UK)  Volume (Liquid)  5.682 × 10^{4} m^{3} 
pint (USA)  Volume (Dry)  5.517 × 10^{4} m^{3} 
pint (USA)  Volume (Liquid)  4.735 × 10^{4} m^{3} 
pound  Weight  0.4536 kg 
poundforce  Force  4.448 N 
rad / röntgen  Radiation Dosage  1.00 × 10^{2} J kg^{1} 
second  Angle  4.848 × 10^{6} rad 
square foot  Area  9.290 × 10^{2} m^{2} 
square inch  Area  6.452 × 10^{4} m^{2} 
square mile  Area  2.590 × 10^{6} m^{2} 
square yard  Area  0.8361 m^{2} 
therm  Energy  1.055 × 10^{8} J 
ton (UK)  Weight  1.016 × 10^{3} kg 
tonne (metric ton)  Weight  1000 kg 
tropical year  Time  3.156 × 10^{7} s 
yard  Length  0.9144 m 
US Metric Association
The USA is the largest country not using the metric system. This site has a lot of useful information.