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 internationally agreed units covering 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 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