Introduction to the metric system

SI units in everyday use
Quantity Unit Symbol Length metre m Mass kilogram kg Time second s Temperature degree Celsius °C Potential difference volt V Energy joule J Power watt W Pressure pascal Pa

Metric prefixes in everyday use
Text	Symbol	Factor	Power
exa	E	7018100000000000000♠1000000000000000000	1018
peta	P	7015100000000000000♠1000000000000000	1015
tera	T	7012100000000000000♠1000000000000	1012
giga	G	7009100000000000000♠1000000000	109
mega	M	7006100000000000000♠1000000	106
kilo	k	7003100000000000000♠1000	103
hecto	h	100	102
deca	da	10	101
(none)	(none)	1	100
deci	d	0.1	10−1
centi	c	0.01	10−2
milli	m	0.001	10−3
micro	μ	6994100000000000000♠0.000001	10−6
nano	n	6991100000000000000♠0.000000001	10−9
pico	p	6988100000000000000♠0.000000000001	10−12
femto	f	6985100000000000000♠0.000000000000001	10−15
atto	a	6982100000000000000♠0.000000000000000001	10−18

Non-SI everday units acceptable for use with SI
Quantity Unit Symbol Time hour (3600 s) minute (60 s) h min Mass tonne (1000 kg) t Volume litre (0.001 m3) L or l Area hectare (10,000 m2) ha Pressure bar (100 kPa) bar

The first recorded proposal for what is now known as the metric system was by John Wilkins, a Fellow of the Royal Society, in 1668. The metric system was further developed during the French Revolution to replace the various measures previously used in France. The metre (also spelled "meter" in some dialects of English) is the unit of length in the metric system and was originally based on the dimensions of the earth, as far as it could be measured at the time. The litre (also spelled "liter"), is the unit of volume and was defined as one thousandth of a cubic metre. The metric unit of mass is the kilogram and it was defined as the mass of one litre of water. The metric system was, in the words of French philosopher Marquis de Condorcet, "for all people for all time".

The metric system has names to cover different ranges of the same measure. Instead of using names based on the context of the measure, the metric system mainly uses names made by adding prefixes, such as kilo- or milli-, as decimal multipliers to the base unit names. Thus, one kilogram is 1000 grams and one kilometre is 1000 metres.

During the nineteenth century the metric system was adopted by both the worldwide scientific community and many countries as the system of measurement. It therefore became truly international. Until 1875 the French government owned the prototype metre and kilogram, but in that year the Convention of the metre was signed and control of the standards relating to mass and length passed on to a trio of inter-government organisations.

In 1960 the metric system was extensively revised to form the International System of Units, abbreviated to SI.

On the eve of the French Revolution, France had an estimated quarter of a million different units of measurement. In many cases the value of a unit differed from town to town and even from trade to trade even though they might have the same name. While certain standards, such as the pied du roi (the King's foot) had a degree of pre-eminence and were used by (scientists), many traders used their own measuring devices. This gave scope for fraud and hindered commerce and industry. The metric system was designed to replace this confusion with a radical new system with fixed values.

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