The Metric System of Measurements
The Metric System of Measurements or more properly the International System of Units (in French the Système international d'unités or SI) was formally established in 1960 by the General Conference on Weights and Measures (CGPM), but its origins date back to the sixteenth century. The CGPM represents 52 member states and 26 further associate members. At the time of establishment, the SI system had 6 base units  the meter, kilogram, second, ampere, degree kelvin (subsequently renamed the "kelvin") and candela and 22 derived units. More units have subsequently been added.
For more information see the NIST Guide for the use of the International System of Units.
Base Units
Meter (m) for length: The length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second. Kilogram (kg) for mass: Equal to the mass of the international prototype of the kilogram. Second (s) for time: The duration of 9,192,631,770 periods of radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. Ampere (A) for electric current: A constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular crosssection, and placed 1 meter apart in a vacuum, would produce between these conductors a force equal to 2 x 10^{7} newton per meter of length. Kelvin (K) for thermodynamic temperature: The fraction 1/273.16 of the thermodynamic temperature of the triple point of water. Candela (cd) for luminous intensity: 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. Mole (mol) for quantity of matter: The amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.
NonSI Units Commonly Used
Ångström (Å) for measure of displacement: Equal to 0.0000000001 meter (10^{10}m). Used to express wavelengths of visible light, UV light, gamma rays, and xrays.
Liter (L) for volume: Equal to 1000 cm^{3.}
Derived Quantity 
Name 
Symbol

Expression in Terms of Other SI Units

Expression in Terms of SI Base Units

plane angle 
radian 
rad 
 
m·m^{1}=1 
solid angle 
steradian 
sr 
 
m^{2}·m^{2}=1 
frequency 
hertz 
Hz 
 
s^{1} 
force 
newton 
N 
 
m·kg·s^{2} 
pressure, stress 
pascal 
Pa 
N/m^{2} 
m^{1}·kg·s^{2} 
energy, work, quantity of heat 
joule 
J 
Nm 
m^{2}·kg·s^{2} 
power, radiant flux 
watt 
W 
J/s 
m^{2}·kg·s^{3} 
electric charge, quantity of electricity 
coulomb 
C 
 
s·A 
electric potential, difference, electromotive force 
volt 
V 
W/A 
m^{2}·kg·s^{3}·A^{1} 
capacitance 
farad 
F 
C/V 
m^{2}·kg^{1}·s^{4}·A^{2} 
electric conductance 
siemens 
S 
A/V 
m^{2}·kg^{1}·s^{3}·A^{2} 
electric resistance 
ohm 
W 
V/A 
m^{2}·kg·s^{3}·A^{2} 
magnetic flux 
weber 
Wb 
Vs 
m^{2}·kg·s^{2}·A^{1} 
magnetic flux density 
tesla 
T 
Wb/m^{2} 
kg·s^{2}·A^{1} 
inductance 
henry 
H 
Wb/A 
m^{2}·kg·s^{2}·A^{2} 
Celsius temperature 
degree Celsius 
°C 
 
K 
luminous flux 
lumen 
lm 
cd·sr 
m^{2}·m^{2}·cd = cd 
illuminance 
lux 
lx 
lm/m^{2} 
m^{2}·m^{4}·cd = m^{2}·cd 
activity (of a radionuclide) 
becquerel 
Bq 
 
s^{1} 
absorbed dose, specific energy (imparted), kerma 
gray 
Gy 
J/kg 
m^{2}·s^{2} 
dose equivalent 
sievert 
Sv 
J/kg 
m^{2}·s^{2} 
catalytic activity 
katal 
kat 
 
s^{1}mol 
SI Prefixes
Prefix 

Symbol 
Factor 
Decimal

Yocto 
Septillion 
y 
10^{24} 

0.000000000000000000000001

Zepto 
Sextillion 
z 
10^{21} 

0.000000000000000000001

Atto 
Quintillionth 
a 
10^{18} 

0.000000000000000001

Femto 
Quadrillionth 
f 
10^{15} 

0.000000000000001

Pico 
Trillionth 
p 
10^{12} 

0.000000000001

Nano 
Billionth 
n 
10^{9} 

0.000000001

Micro 
Millionth 
µ 
10^{6} 

0.000001

Milli 
Thousandth 
m 
10^{3} 

0.001

Centi 
Hundredth 
c 
10^{2} 

0.01

Deci 
Tenth 
d 
10^{1} 

0.1

Base Unit 
One 

10^{0} 
1

Deca 
Ten 
da 
10^{1} 
10


Hecto 
Hundred 
h 
10^{2} 
100


Kilo 
Thousand 
k 
10^{3} 
1 000


Mega 
Million 
M 
10^{6} 
100 0000


Giga 
Billion 
G 
10^{9} 
1000 000 000


Tera 
Trillion 
T 
10^{12} 
1000 000 000 000


Peta 
Quadrillion 
P 
10^{15} 
1 000 000 000 000 000


Exa 
Quintillion 
E 
10^{18} 
1 000 000 000 000 000 000


Zetta 
Sextillionth 
Z 
10^{21} 
1 000 000 000 000 000 000 000


Yotta 
Septillionth 
Y 
10^{24} 
1 000 000 000 000 000 000 000 000


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