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SI Base Units

Seven units that all other metric units are based on.

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SI Base Units

How long is a yard? 

It depends on who, and when, you asked the question. Today, we have a standard definition of the yard, which you can see marked on every football field. If you move the ball ten yards, you get a first down and it doesn’t matter whether you are playing in Los Angeles, Dallas, or Green Bay. But at one time that yard was arbitrarily defined as the distance from the tip of the king’s nose to the end of his outstretched hand. Of course, the problem there is simple: new king, new distance (and then you have to remark all those football fields).

SI Base Units

All measurements depend on the use of units that are well known and understood. The English system of measurement units (inches, feet, ounces, etc.) are not used in science because of the difficulty in converting from one unit to another. The metric system is used because all metric units are based on multiples of 10, making conversions very simple. The metric system was originally established in France in 1795. The International System of Units is a system of measurement based on the metric system. The acronym SI is commonly used to refer to this system and stands for the French term, Le Système International d’Unités. The SI was adopted by international agreement in 1960 and is composed of seven base units, as shown in Table below.

SI Base Units of Measurement
Quantity SI Base Unit Symbol
Length meter m
Mass kilogram kg
Temperature kelvin K
Time second s
Amount of a Substance mole mol
Electric Current ampere A
Luminous Intensity candela cd

The first five units are frequently encountered in chemistry. All other measurement quantities, such as volume, force, and energy, can be derived from these seven base units.

The meter standard

Credit: Courtesy of US Department of Commerce
Source: http://commons.wikimedia.org/wiki/File:US_National_Length_Meter.JPG
License: CC BY-NC 3.0

Meter standard.[Figure1]

The kilogram standard

Credit: Courtesy of National Institute of Standards and Technology
Source: http://commons.wikimedia.org/wiki/File:Standard_kilogram,_2.jpg
License: CC BY-NC 3.0

Kilogram standard[Figure2]

The map in Figure below shows the adoption of the SI units in countries around the world. The United States has legally adopted the metric system for measurements, but does not use it in everyday practice. Great Britain and much of Canada use a combination of metric and imperial units.

Map of the world that uses the metric system

Credit: User:Canuckguy/Wikipedia and User:AzaToth/Wikimedia Commons
Source: http://commons.wikimedia.org/wiki/File:Metric_system_adoption_map.svg
License: CC BY-NC 3.0

Areas of world using metric system.[Figure3]


  • The SI system is based on multiples of ten.
  • There are seven basic units in the SI system.
  • Five of these units are commonly used in chemistry.


  1. What does SI stand for?
  2. When was this system adopted by the international community?
  3. Which of the units are commonly used in chemistry?

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Image Attributions

  1. [1]^ Credit: Courtesy of US Department of Commerce; Source: http://commons.wikimedia.org/wiki/File:US_National_Length_Meter.JPG; License: CC BY-NC 3.0
  2. [2]^ Credit: Courtesy of National Institute of Standards and Technology; Source: http://commons.wikimedia.org/wiki/File:Standard_kilogram,_2.jpg; License: CC BY-NC 3.0
  3. [3]^ Credit: User:Canuckguy/Wikipedia and User:AzaToth/Wikimedia Commons; Source: http://commons.wikimedia.org/wiki/File:Metric_system_adoption_map.svg; License: CC BY-NC 3.0

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