# International System of Units (SI)

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 = 1024 yotta Y
1 000 000 000 000 000 000 000 = 1021 zetta Z
1 000 000 000 000 000 000 = 1018 exa E
1 000 000 000 000 000 = 1015 peta P
1 000 000 000 000 = 1012 tera T
1 000 000 000 = 109 giga G
1 000 000 = 106 mega M
1 000 = 103 kilo k
100 = 102 hecto h
10 = 101 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.

SI derived units
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 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/s2
Frequency (of a periodic phenomenon) hertz Hz 1/s
Illuminance lux lx lm/m2
Luminous flux lumen lm cd•sr
Magnetic flux weber Wb V•s
Magnetic flux density tesla T Wb/m2