S.I. Units

From DT Online


The modern metric system is the International System of Units (French: Système International d'Unités, SI). The system was published in 1960 and was based on the metre-kilogram-second system of units (MKS). There are now seven internationally agreed units plus units derived from them to cover all aspects of technology together with a set of prefixes which serve as multipliers or fractions of them.

The preferred prefixes multiply or divide the unit names by 1000 (or 10-3) thus, 10mm is preferred to a centimetre, especially in construction and engineering.

Note: Units named after people use capital letters for the letter symbol - but not for the unit name itself.

Measurement of Length
Prefix Name Symbol 10n Decimal Description
kilometre km 103 1000 A thousand metres
metre m 100 1 A metre
centimetre cm 10-2 0.01 A hundredth of a metre
millimetre mm 10-3 0.001 A thousandth of a metre
micrometre μm 10-6 0.000001 A millionth of a metre

Units used in Mechanics
Prefix Name Symbol 10n Decimal Description
gram g 10-3 1/1000 Originally the weight of a cubic centimetre of pure water but now defined as one-thousandth of a kilogram
kilogram kg 100 1 S.I. Base Unit of Mass - equal to the mass of the International Prototype of the Kilogram
tonne t 103 1000 Known as Metric Ton in USA (i.e. equivalent to approximately 2,204.6lbs whereas Imperial Ton = 2240lbs)
megatonne Mt 109 1,000,000,000 Used for the energy of nuclear explosions and other similar events.
Newton N 100 1 S.I. Base Unit of Force named after Sir Isaac Newton - the amount needed to accelerate a 1kg mass at 1 metre per second each second (1N = 1kg X ms2)
Newton Metre Nm 100 1 S.I. Base Unit of Torque or Moment - resulting from applying a force of one newton perpendicularly to a moment arm which is one metre long (1Nm). It is calculated as for a joule but is preferred for Torque to avoid confusion with Work and Energy.
Joule J 100 1 S.I. Base Unit of Work or Energy named after James Prescott Joule - the amount needed to apply a newton over the distance of a metre (1J = 1Nm)
Pascal Pa 100 1 S.I. Base Unit of Pressure or Stress named after Blaise Pascal - the pressure exerted by applying a newton over a square metre (1Pa = 1Nm2)

Units used in Electronics
Ohms (resistance) - named after Georg Simon Ohm
Prefix Name Symbol 10n Decimal Description
mega-ohm 106 1000000 A million ohms
kilo-ohm (or K) 103 1000 A thousand ohms
ohm Ω (or R) 100 1 An ohm

The unit or its prefix may be used to replace a decimal point - e.g. 2.2mega-ohm = 2M2; 2.2kilo-ohm = 2K2; and 2.2ohm = 2R2.

Ampere (electric current)' - named after André-Marie Ampère
Prefix Name Symbol 10n Decimal Description
ampere A 100 1 Amp
milli-amp mA 10-3 0.001 A thousandth
Farads (electric capacitance) - named after Michael Faraday
Prefix Name Symbol 10n Decimal Description
farad F 100 1 A Farad
micro-Farad μF 10-6 0.000001 A millionth
nano-Farad nF 10-9 0.000000001 A thousand millionth
pico-Farad pF 10-12 0.000000000001 A trillionth