Custom Search

SI System

An introduction to the SI-system

Sponsored Links

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 a small agency in Paris, the International Bureau of Weights and Measures (BIPM, for Bureau International des Poids et Mesures).

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.

Quantity Name of Unit Symbol
Length meter m
Mass kilogram kg
Time second s
Electrical current ampere A
Thermodynamic temperature Kelvin K
Luminous intensity candela cd
Amount of substance mole mol

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.

Quantity Name of Unit Symbol Expression in terms of SI base units Expression 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 w m2 kg s-3 A-2 V/A
Force Newton N kg ms-2  
Frequency hertz Hz s-1  
Luminance 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 bequerel Bq s-1  
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. 

Quantity Description Symbol Expression 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 columb 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 columb 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-1
molar energy joule per mole J/mol m-2 kg s-2 mol-1
molar entropy (or molar heat capacity) joule per mole Kelvin J/(mol 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) kilopascal 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

Sponsored Links

Related Topics

  • Basics - Basic Information as SI-system, Unit converters, Physical constants...

Sponsored Links

Related Documents

Engineering News

Engineering Standards