S.I. Units

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.

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

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Measurement of Length

Metre
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	MΩ	106	1000000	A million ohms
kilo-ohm	kΩ (or K)	103	1000	A thousand ohms
ohm	Ω (or R)	100	1	An ohm
Note: 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