The standart units of measurement in science are part of a measuring system called the International System (SI). The letters are reversed in the symbol because they are taken from the French name Le Systeme International d'Unites. SI is used by all scientists throughout the world. It is a modern version of the metric system. The people in most countries use SI in everyday life or are in the process of converting to SI.
One important feature of SI is its simplicity. Seven base units are the foundation of the International System. These units are shown below:
SI Base Units |
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Quantity |
Unit |
Unit Symbol |
Length |
meter |
m |
Mass |
kilogram |
kg |
Time |
second |
s |
Electric current |
ampere |
A |
Thermodynamic temperature |
kelvin |
K |
Amount of substance |
mole |
mol |
Luminous intensity |
candela |
cd |
More detailed definitions of these units can be found here.
In SI, prefixes are added to the base units to obtain different units of a convenient size for measuring larger or smaller quantities. A kilometer is one thousand meter while a millimeter is one thousandth of a meter. A kilometer is convenient unit for measuring distance between cities. Millimeters are useful for measuring the thickness of a board. The table of commonly used SI prefixes is available here.