
<< Back 
International System of Units (SI)

The modern form of the metric system, which has been adopted by the United States and most other nations. Note: The SI is constructed from seven base units for independent physical quantities. Tables showing these values are included below and are current as of Fall 1995.
SI Prefixes. The common metric prefixes are:
Multiplication Factor 
Prefix Name 
Prefix 
Symbol 
1 000 000 000 000 000 000 000 000 = 
10^{24} 
yotta 
Y 
1 000 000 000 000 000 000 000 = 
10^{21} 
zetta 
Z 
1 000 000 000 000 000 000 = 
10^{18} 
exa 
E 
1 000 000 000 000 000 = 
10^{15} 
peta 
P 
1 000 000 000 000 = 
10^{12} 
tera 
T 
1 000 000 000 = 
10^{9} 
giga 
G 
1 000 000 = 
10^{6} 
mega 
M 
1 000 = 
10^{3} 
kilo 
k 
100 = 
10^{2} 
hecto 
h 
10 = 
10^{1} 
deka 
da 
0.1 = 
10^{1} 
deci 
d 
0.01 = 
10^{2} 
centi 
c 
0.001 = 
10^{3} 
milli 
m 
0.000 001 = 
10^{6} 
micro 

0.000 000 001 = 
10^{9} 
nano 
n 
0.000 000 000 001 = 
10^{12} 
pico 
p 
0.000 000 000 000 001 = 
10^{15} 
femto 
f 
0.000 000 000 000 000 001 = 
10^{18} 
atto 
a 
0.000 000 000 000 000 000 001 = 
10^{21} 
zepto 
z 
0.000 000 000 000 000 000 000 001 = 
10^{24} 
yocto 
y 
SI Base Units:
Quantity
 Unit Name
 Unit Symbol 
length 
meter 
m 
mass 
kilogram 
kg 
time 
second 
s 
electric current 
ampere 
A 
thermodynamic temperature 
kelvin 
K 
amount of substance 
mole 
mol 
luminous intensity 
candela 
cd 
SI derived units. Derived units are formed by combining base units and other derived units according to the algebraic relations linking the corresponding quantities. The symbols for derived units are obtained by means of the mathematical signs for multiplication, division, and use of exponents. For example, the SI unit for velocity is the meter per second (m/s or m•s^{1}), and that for angular velocity is the radian per second (rad/s or rad•s^{1}). Some derived SI units have been given special names and symbols, as listed in this table:
Quantity 
Unit Name 
Unit Symbol 
Expression in Terms of Other SI Units 
Absorbed dose, specific energy imparted, kerma, absorbed dose index 
gray 
Gy 
J/kg 
Activity (of a radionuclide) 
becquerel 
Bq 
1/s 
Celsius temperature 
degree Celsius 
C 
K 
Dose equivalent 
sievert 
Sv 
J/kg 
Electric capacitance 
farad 
F 
C/V 
Electric charge, quantity of electricity 
coulomb 
C 
A•s 
Electric conductance 
siemens 
S 
A/V 
Electric inductance 
henry 
H 
Wb/A 
Electric potential, potential difference, electromotive force 
volt 
V 
W/A 
Electric resistance 
ohm 

V/A 
Energy, work, quantity of heat 
joule 
J 
N•m 
Force 
newton 
N 
kg•m/s^{2} 
Frequency (of a periodic phenomenon) 
hertz 
Hz 
1/s 
Illuminance 
lux 
lx 
lm/m^{2} 
Luminous flux 
lumen 
lm 
cd•sr 
Magnetic flux 
weber 
Wb 
V•s 
Magnetic flux density 
tesla 
T 
Wb/m^{2} 
Plane angle 
radian 
rad 
m/m 
Power, radiant flux 
watt 
W 
J/s 
Pressure, stress 
pascal 
Pa 
N/m^{2} 
Solid angle 
steradian 
sr 
m^{2}/m^{2} 


