The metric system is the modern approach to weights and measures. Although it is based on scientific principles, it is very flexible and, once the simple basic rules have been grasped, it is easy to use for everyday purposes. The ease of use has been a key factor in its adoption worldwide.

A system of units

Metric units form a "system" which means it can be readily applied to any physical quantity. A wide range of uses are possible ranging from home cooking & DIY, weather reporting, electrical applications, motoring to radiotherapy.

Traditional units are generally ad hoc collections of units that do not have a consistent relationship with each other. For example with imperial there are both avoirdupois and troy units for measuring mass. Volume in imperial may be based on cubic feet or inches for solids but gallons and pints for liquids. Imperial uses a wide range of awkward number systems (e.g. 8 pints in a gallon, 14 pounds in a stone etc).

In the international system there is a single unit for each physical phenomenon Thus grams are used for the weight (strictly speaking, the "mass") of anything - whether gold, herbs or garden fertilizer. Similarly litres are equally applicable to volumes of cooking oil, the capacity of a refrigerator, the amount of air in a room, or the wine production of a vineyard.

Units in the international system follow physical principles. For example in an electrical circuit the power consumed is equal to the voltage drop times the current passing.

Power = voltage drop × current

The unit of power (watt), by the same principle, is equal to one volt times one amp

1 W = 1 V × 1 A

Similarly there is a straightforward mechanical definition for the watt.

This idea of units fitting together systematically or coherently was pioneered by British scientists in the 1860s.

Metric values for water make everyday use easy

Water is essential for life and its properties affect everybody. The freezing point of water is important for weather, for preserving food and for motoring. Similarly the boiling point of water is important for cooking for many mechanical systems. The temperature scales in metric (whether kelvin (K) or degrees Celsius(°C))have an interval of 100 between the freezing point and boiling point of water.

A litre of water weighs exactly one kilogram

There is also a convenient match between the kilogram and litre of water. As John Fraser MP said in a House of Commons debate in 1989 “A cube with a 10 cm base contains exactly 1 000 cm³ and 1 000 mL. Filled with water, it weights exactly 1 kg… It is a beautiful and delightful system with a particular simplicity”.

Decimal numbers make calculation simple

The metric system was intended from the outset to be based on decimal numbers. People think and calculate most easily in decimal and money is also decimal. Simple calculations using metric are easily done in the head. More complex cases are readily handled with a pen and paper or with a calculator.

Eeryday units

Units such as the metre, kilogram, litre, volt, watt and degree Celsius are commonly used in Britain . A few examples are given below:


metres for signage	a kilogram of sugar

a litre of fruit juice	a 1.5 volt battery

30 watt speaker	a weather forecast in degrees Celsius

International Development and Usage of Metric

The metric system started in France and was rapidly adopted in other European countries during the 19th century. Since then it has been adopted in most of the world. The development of the metric system was made truly international by the signing of the Convention du Mètre (Metre Convention) in 1875; a diplomatic treaty between an initial set of 17 countries. Britain signed the Convention in 1884 and there are now 51 member countries.

In 1960 the 11th General Conference on Weights and Measures adopted the name International System of Units (Système International d'Unités or SI) for the recommended practical system of units of measurement. SI consists of different classes of units:

seven independent base units. These include the metre (m), the kilogram (kg), the second (s), the ampere (A), the kelvin (K), the mole (mol) and the candela(cd).

a set of derived units which are defined in terms of base units. Examples include the watt (W), the volt (V), the square metre (m²) and the degree celsius (°C).

There is also a standard set of prefixes which allows metric units to describe a wide range of quantities from very small to very large. Common examples of prefixes are:

micro- ( µ-) meaning millionth e.g. microvolt ( µ V)

milli- (m-) meaning thousandth e.g. millimetre (mm)

kilo- (k-) meaning thousand e.g. kilogram (kg)

mega- (M-) meaning million e.g. megawatt (MW)

The writing of prefixes and units is standardised internationally. Furthermore the use of standard prefixes and units means that pronunciation should follow straightforward rules.

The Metre Convention has also established the International Bureau of Weights and Measures (Bureau international des poids et mesures, BIPM ) as an international centre for metrology in Sèvres near Paris . BIPM is responsible for providing a scientific basis for a single, coherent set of units and for coordinating comparisons of national measurement standards.

From the outset the metric system spread internationally, aided by international participation in its development. The Commonwealth and the United States kept traditional measurement much longer than other countries. After a century of recognising the benefits of metric, Britain finally announced that it would adopt metric units in 1965. Commonwealth countries followed this lead and in the 1970s Australia, New Zealand and South Africa successfully converted from imperial to metric. However, Britain and the USA have yet to complete the changeover.