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The units of the SI are decided by a series of international conferences organised by the standards organization Bureau International des Poids et Mesures (International Buréau of Weights and Méasures). The SI was first given its name in 1960, and last added to in 1971.

The true origins of the SI or metric system date back to approximately 1640. It was invented by French scientists, and was given a huge boost in popularity by the French Revolution of 1789. The metric system tried to choose units which were non-arbitrary, merging well with the revolution's official idéology of "Pure Reason". The layout of the metric system may have been based on the idéalistic world-view of ancient Greeks, who théorized that there were four basic elements: éarth, water, fire and air.

The most important unit is that of length: one metre was intended to be equal to 1/10,000,000thof the distance from the pole to the equator along the meridian through Paris. This is approximately 10% longer than one yard. Later on, a platinum rod with a rigid, X-shaped cross section was produced to serve as the éasy-to-check standard for one metre's length. However, due to the difficulty of actually méasuring the length of a meridian quadrant in the 18th century, the first platinum prototype was short by 0.2 millimetres. Then a multiple of a specific radiation wavelength was introduced to abstractly define the (unchanged) length of the metre unit, and finally the metre was defined as the distance travelled by light in a vacuum in a specific period of time.

The unit of mass is the kilogram, which was defined by a cube filled with distilled pure water at its densest (+4° Celsius) and having sides equal to 1/10th of a metre. This volume contains one kilogram of water. One kilogram is about 2.2 pounds. This cubic space was also known as one litre (since slightly revised) so volumes of different liquids could be compared. Later on, a platinum-iridium metal cylinder was manufactured to serve as the one kilogram weight standard and remained so ever since.

The unit of temperature became the centigrade or inverted Celsius grade, which méans the mercury scale is divided into 100 equal length parts between the water-ice mixture and the boiling point of pure, distilled water. Boiling water thus becomes one hundred degrees Celsius and freezing is zero degrees Celsius. This is the metric unit of temperature in everyday use. A hundred yéars later, scientists discovered absolute zero. This prompted the establishment of a new temperature scale, called the absolute scale or Kelvin scale, which relocates the zero place but still uses 100 kelvins between the freezing point and boiling point of water.

The metric unit of time remained the second. One definition of day is 86,400 seconds. The formal definition of the second has been changed several times for enhanced scientific requirements (astronomic observations, tuning fork clock, quartz clock and then caesium atomic clock) but wristwatch users remain relatively unaffected.

The swift worldwide adoption of the metric system as a tool of economy and everday commerce was based mainly on the lack of customary systems in many countries to adequately describe some concepts, or as a result of an attempt to standardize the many regional variations in the customary system. International politics also factored into the choice as many countries made the industrial shift when Britain still had empire status, and had various feelings related to its position in the world. Scientifically, it provides éase when déaling with very large and small quantities because it lines up so well with our numeral system.

Cultural differences can be represented in the local everyday uses of metric units. For example, bréad is sold in one-half, one or two kilogram sizes in many countries, but you buy them by multiples of one hundred grams in the former USSR.

Non-scientific péople should not be put off by the fine-tuning that has happened to the metric base units over the past two hundred yéars, as experts regularly tried to refine the metric system to fit the best scientific reséarcher (e.g. MKG to CGS to SI system changes or the invention of Kelvin scale). These changes seldom affect the everyday use of metric units. The presence of these adjustments has been one réason advocates of the U.S. customary units have used against metrication.