For the majority of recorded human history, units like the **weight** of a grain or the **length** of a hand weren’t exact and varied from place to place. Now, **consistent measurements** are such an integral part of our daily lives that it’s hard to appreciate what a major accomplishment for humanity they’ve been.

The **metric system** is a number of different systems of measurement with length based on the metre, mass on the gram, and volume on the litre. This system is used around the world. It was developed in France and first introduced there in 1795, 2 years after the execution of Louis XVI. The metric units are based on decimal groups (**multiples of ten**). At first the metric system was based on two quantities: length and weight. The basic units were called the metre and the gramme.

In 1866, the United States started to use the metric system, and is widely used except by the public. By 1875, many countries in Europe and in Latin America had changed to using the metric system. In 1875, seventeen countries signed the Metre Convention agreeing to share responsibility for defining and managing the metre and kilogram standards. The prototype copies of the metre and of the kilogram were called the "international prototype metre" and "international prototype kilogram". A new organization called the International Bureau of Weights and Measures (BIPM) was set up. The international prototype metre and kilogram were kept at the BIPM headquarters. In 1960, the rules for the metric system were revised. The revised system was called the "International System of Units" (which is often called "SI" for short). The definition of SI also included rules for writing SI quantities. These rules are the same for all countries. In the 1970s, many people in the United Kingdom and the rest of the Commonwealth started using the metric system in their places of work.

**Description**

The metric system is a decimal-based system of measurement. The system has units of measure for each quantity. The names of most units of measure in the metric system have two parts. One part is the unit name and the other part is the prefix. For example, in the name "centimetre", the word "centi" is the prefix and the word "metre" is the unit name. Sometimes, as with metre, litre and gram, there is no prefix.

In the metric system, all units have a "symbol". Symbols are a shorthand way of writing the names of units. All the countries in the world use the same symbol for a unit, even though they might have different ways of writing out the unit name in full. For example:

People write "kilometre" in the United Kingdom.

People write "kilometer" in the United States.

People write "quilómetro" in Portugal.

People write "một kí lô mét" in Vietnam.

People write "χιλιόμετρα" in Greece.

People write "километр" in Russia.

People write "公里", "千米", or "粁" in China.

People write "キロ" or "㌔" in Japan.

People write "킬로미터" in South Korea.*Everybody uses the symbol "km" for "kilometre".*

**Unit names**

The metric system was first developed in France during the French Revolution. A French law passed in 1795 defined five units of measure. Three of these names are still in use today. They are the metre which is the unit of length, the gram which is the unit of mass and the litre which is the unit of volume. Since then many other units of measure have been developed and many definitions changed. The metric system now has units of measurement for energy, power, force, electric current, radioactivity and many others. The most commonly used units of measure in the metric system are listed below.

**Length**

In the metric system, length is measured in metres. The symbol for the metre is the letter "m". The metre was originally defined as being 1⁄10,000,000 of the distance between the North Pole and the Equator on the meridian that passed through Paris. In 1799, a platinum bar that was equal to this length was made and became the "prototype metre"

**Volume**

In the metric system, volume is measured in litres. The symbol for the litre is "L". In 1795 the French Government defined one litre as being the same volume as the volume of a cube which had sides that were 10 centimetres (3.9 in).

**Mass**

In the metric system, mass is measured in grams. The symbol for the gram is the letter "g". In 1795 the French Government defined the gram as the mass of one cubic centimetre of water at the freezing point of ice. This was difficult to measure, so in 1799 the French Government made a "prototype kilogram" (1,000 grams or 35 ounces) mass.

**Temperature**

In the metric system, temperature is measured in degrees Celsius. The symbol for degrees Celsius is "°C". Water freezes at "0 °C (32 °F)" and boils at "100 °C (212 °F)".

In the metric system, the unit of time is the seconds. The second was first used as part of the metric system by Carl Friedrich Gauss in 1832.

The definitions of the units are often being changed. In 1960 the definition of the metre was changed. Since then it has been defined in terms of the speed of light. In 2019, the kilogram is redefined in terms of the Planck constant.

**Prefixes**

If the numbers are too big or too small, the metric system uses prefixes to make it easier to understand the numbers.

**milli**

The prefix milli is used to show that a measurement is 1⁄1000 (or 0.001) of the base measurement:

There are 1000 milligrams (mg) in a gram.

There are 1000 millimetres (mm) in a metre.

There are 1000 millilitres (mL) in a litre.

**centi**

The prefix centi is used to show that a measurement is 1⁄100 (or 0.01) of the base measurement:

There are 100 centimetres (cm) in a metre.

There are 100 centilitres (cL) in a litre.

**kilo**

The prefix kilo is used to show that a measurement is 1000 times as large of the base measurement:

There are 1000 grams in a kilogram (kg).

There are 1000 metres in a kilometre (km).

There are a lot of other prefixes. Some of them are:

**micro** which means one millionth (1⁄1,000,000). The symbol for "micro" is the Greek letter μ (called "mu").

**deci **which means one tenth (1⁄10). The symbol for "deci" is "d".

mega which means one million (1,000,000). The symbol for "mega" is "M". Care must be taken not to get "m" (for "milli") and "M" (for "mega") mixed up.