
-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 cross-section, 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 1012 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.
Non-SI Units Commonly Used
Ångström (Å) for measure of displacement: Equal to 0.0000000001 meter (10-10m). Used to express wavelengths of visible light, UV light, gamma rays, and x-rays.
Liter (L) for volume: Equal to 1000 cm3.
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 |
- |
m2·m-2=1 |
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 |
Nm |
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 |
capacitance |
farad |
F |
C/V |
m-2·kg-1·s4·A2 |
electric conductance |
siemens |
S |
A/V |
m-2·kg-1·s3·A2 |
electric resistance |
ohm |
W |
V/A |
m2·kg·s-3·A-2 |
magnetic flux |
weber |
Wb |
Vs |
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 |
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 |
sievert |
Sv |
J/kg |
m2·s-2 |
catalytic activity |
katal |
kat |
- |
s-1mol |
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 |
|
100 |
1
|
Deca |
Ten |
da |
101 |
10
|
|
Hecto |
Hundred |
h |
102 |
100
|
|
Kilo |
Thousand |
k |
103 |
1 000
|
|
Mega |
Million |
M |
106 |
100 0000
|
|
Giga |
Billion |
G |
109 |
1000 000 000
|
|
Tera |
Trillion |
T |
1012 |
1000 000 000 000
|
|
Peta |
Quadrillion |
P |
1015 |
1 000 000 000 000 000
|
|
Exa |
Quintillion |
E |
1018 |
1 000 000 000 000 000 000
|
|
Zetta |
Sextillionth |
Z |
1021 |
1 000 000 000 000 000 000 000
|
|
Yotta |
Septillionth |
Y |
1024 |
1 000 000 000 000 000 000 000 000
|
|
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