The International System of Units (abbreviated as SI from the French language name Système International d'Unités) is the modern revision of themetric system. It is the world's most widely used system of units, both in everyday commerce and in science. The SI was developed in 1960 from the metre-kilogram-second (MKS) system, rather than the centimetre-gram-second (CGS) system, which, in turn, had many variants. During its development the SI also introduced several newly named units that were previously not a part of the metric system. The original SI units for the six basic physical quantities were:

Base quantity

Base unit

Symbol

Current SI constants

New SI constants (proposed)

time

second

s

hyperfine splitting in Cesium-133

same as current SI

length

metre

m

speed of light in vacuum, c

same as current SI

mass

kilogram

kg

mass of International Prototype Kilogram (IPK)

Planck's constant, h

electric current

Ampere

A

permeability of free space, permittivity of free space

charge of the electron, e

temperature

Kelvin

K

triple point of water, absolute zero

Boltzmann's constant, k

amount of substance

mole

mol

molar mass of Carbon-12

Avogadro constant N_{A}

luminous intensity

candela

cd

luminous efficacy of a 540 THz source

same as current SI

The mole was subsequently added to this list and the degree Kelvin renamed the kelvin.

There are two types of SI units, base units and derived units. Base units are the simple measurements for time, length, mass, temperature, amount of substance, electric current and light intensity. Derived units are constructed from the base units, for example, the Watt, i.e. the unit for power, is defined from the base units as m^{2}·kg·s^{−3}. Other physical properties may be measured in compound units, such as material density, measured in kg/m^{3}.

Converting prefixes

The SI allows easy multiplication when switching among units having the same base but different prefixes. To convert from metres to centimetres it is only necessary to multiply the number of metres by 100, since there are 100 centimetres in a metre. Inversely, to switch from centimetres to metres one multiplies the number of centimetres by 0.01 or divide centimetres by 100.

Length

A 2-metre carpenter's ruler. A ruler or rule is a tool used in, for example, geometry, technical drawing, engineering, and carpentry, to measure lengths or distances or to draw straight lines. Strictly speaking, the ruler is the instrument used to rule straight lines and the calibrated instrument used for determining length is called a measure, however common usage calls both instruments rulers and the special name straightedge is used for an unmarked rule. The use of the word measure, in the sense of a measuring instrument, only survives in the phrase tape measure, an instrument that can be used to measure but cannot be used to draw straight lines. As can be seen in the photographs on this page, a two-metre carpenter's rule can be folded down to a length of only 20 centimetres, to easily fit in a pocket, and a five-metre-long tape measure easily retracts to fit within a small housing.