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SI System

An introduction to the SI metric system.

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The SI system (International System of Units) is the modern metric system of measurement and the dominant system of international commerce and trade. SI units are gradually replacing Imperial and USCS units.

The SI is maintained by the International Bureau of Weights and Measures (BIPM, for Bureau International des Poids et Mesures) in Paris.

The SI system is founded on the

SI Base Units

The core of the SI system is a short list of base units defined in an absolute way without referring to any other units. The base units are consistent with the part of the metric system called the MKS system. The International System of Units (SI) is founded on seven base units.

QuantityName of UnitSymbol
Length metre m
Mass kilogram kg
Time second s
Electrical current ampere A
Thermodynamic temperature kelvin K
Luminous intensity candela cd
Amount of substance mole mole

SI Derived Units with Special Names and Symbols acceptable in SI

Derived units are algebraic combinations of the seven base units and the two supplementary units with some of the combinations being assigned special names and symbols.

QuantityName of UnitSymbolExpression in terms of SI base unitsExpression in terms of other units
Plane angle radian rad    
Solid angle steradian sr    
Adsorbed radiation gray Gy m2 s-2 J/kg
Electrical capacitance farad F m-2 kg-1 s4 A2 C/V
Electrical charge coulomb C A s  
Electrical conductance siemens S m-2 kg-1s3A2 A/V
Electrical inductance henry H m2 kg s-2 A-2  
Electrical potential volt V m2 kg s-3 A-1 W/A
Electrical resistance ohm Ω m2 kg s-3 A-2 V/A
Force newton N kg ms-2  
Frequency hertz Hz s-1  
Illuminance lux lx m-2cd sr lm/m2
Luminous flux lumen lm cd sr  
Magnetic flux weber Wb m2 kg s-2 A-1 V s
Magnetic flux density tesla T kg s-2 A-1 Wb/m2
Power or radiant flux watt W kg m2 s-3 J/s
Pressure pascal Pa kg/(m s2) = (N/m2)  
Radioactivity becquerel Bq s-1  
Temperature relative to 273.15 K degree Celsius °C K
Work, energy, heat joule J m2 kg s-2 N m

SI Derived Units described in Terms of acceptable SI Units

Derived units are algebraic combinations of the seven base units and the two supplementary units with some of the combinations being assigned special names and symbols. 

QuantityDescriptionSymbolExpression in terms of SI base units
acceleration meter per second squared m/s2 m s-2
area square meter m2 m2
coefficient of heat transfer (often used symbol h or U) watt per square meter Kelvin W/(m2 K) kg s-3K-1
concentration (of amount of substance) mole per cubic meter mol/m3 mol m-3
current density (often used symbol r) ampere per square meter A/m2 A m-2
density (mass density) kilogram per cubic meter kg/m3 kg m-3
electrical charge density coulomb per cubic meter C/m3 m-3 s A
electric field strength volt per meter V/m m kg s-3 A-1
electric flux density coulomb per square meter C/m2 m-2 s A
energy density joule per cubic meter J/m3 m-1 kg s-2
force Newton N or J/M m kg s-2
heat capacity joule per Kelvin J/K m2 kg s-2 K-1
heat flow rate (often used symbol Q or q) watt W or J/s m2 kg s-3
heat flux density or irradiance watt per square meter W/m2 kg s-3
luminance candela per square meter cd/m2 cd m-2
magnetic field strength ampere per meter A/m A m-1
modulus of elasticity (or Young's modulus) giga Pascal GPa 10-9 m-1 kg s-2
molar energy joule per mole J/mole m-2 kg s-2 mol-1
molar entropy (or molar heat capacity) joule per mole Kelvin J/(mole K) m-2 kg s-2 K-1 mol-1
moment of force (or torque) Newton meter N m m2 kg s-2
moment of inertia kilogram meter squared kg m2 kg m2
momentum kilogram meter per second kg m/s kg m s-1
permeability Henry per meter H/m m kg s-2 A-2
permitivity farad per meter F/m m-3 kg-1 s4 A2
power kilowatt kW 10-3 m2 kg s-3
pressure (often used symbol P or p) kilo Pascal kPa 10-3 m-1 kg s-2
specific energy joule per kilogram J/kg m2 s-2
specific heat capacity (or specific entropy, often used symbol c,p,cv or s) joule per kilogram Kelvin J/(kg K) m2 s-2 K-1
specific volume cubic meter per kilogram m3/kg m3 kg-1
stress mega Pascal MPa 10-6 m-1 kg s-2
surface tension Newton per meter N/m kg s-2
thermal conductivity (often used symbol k) watt per meter Kelvin W/(m K) m kg s-3 K-1
torque Newton meter N m m2 kg s-2
velocity (or speed) meters per second m/s m s-1
viscosity, absolute or dynamic (often used symbol m) Pascal second Pa s m-1 kg s-1
viscosity, kinematic (often used symbol n) square meter per second m2/s m2 s-1
volume cubic meter m3 m3
wave number 1 per meter 1/m m-1
work (or energy heat, often used symbol W) joule J or N m m2 kg s-2

SI Prefixes

Number Greek Latin
½ hemi semi
1 mono uni
2 di bi
3 tri ter
4 tetra quandri
5 penta quinque
6 hexa sexi
7 hepta septi
8 octa octo
9 ennea nona
10 deca deci
11 hendeca undec
12 dodeca duodec
13 trideca tridec
14 tetradeca quatuordec
15 pentadeca quindec
16 hexadeca sedec
17 heptadeca septendec
20 eicosane vige, vice
30 triaconta trige, trice
40 tetraconta quadrage
50 pentaconta quincuage
60 hexaconta sexage
70 heptaconta septuage
80 octaconta octage
90 enneaconta nonage
100 hecto cente
many poly multi

1024 yotta Y
1021 zetta Z
1018 exa E
1015 peta P
1012 tera T
109 giga G
106 mega M
103 kilo k
102 hecto h
101 deca da
10-1 deci d
10-2 centi c
10-3 milli m
10-6 micro μ
10-9 nano n
10-12 pico p
10-15 femto f
10-18 atto a
10-21 zepto z
10-24 yocto y

The prefixes provide an order of magnitude: Ex.

  • 16600 m = 16.6 103 m = 16.6 km
  • 1 centimeter = 10-2 m
  • 1 millimeter = 10-3 m
  • 1 micrometer = 10-6 m
  • 1 nanometer = 10-9 m
  • 1 mm3 = (10-3 m)3 = 10-9 m3
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