Last edited 04 Mar 2019

#  Introduction

The metric system is a decimalised measurement system used widely throughout the world for weights and measures in everyday life, trade, science and industry. It can be used for example to express the weight of vegetables, the speed of a train, the height of a child, the intensity of light, the volume of water and so on.

One of the benefits of the metric system is its relative ease of understanding and application. Being a decimalised system, it is based on the power of 10 and parts of ten. So, there are 10 millimetres (mm) in a centimetre (cm); 100 centimetres in a metre (m) and 1,000 metres in a kilometre (km).

This can be contrasted with the relative complexity of the Imperial system which was based on irregular bases, eg 12 pence in a shilling, 20 shillings in a pound (240 pence); 12 inches in a foot (ft), three feet in a yard (yd) and 1,760 yards (yds) in a mile.

The imperial system also involved fractions that were more problematic when adding up, eg 1ft 3 3/4in plus 4ft 3 5/8in equals 5ft 7 3/8in. The metric equivalent of this addition is 396mm + 1310mm = 1706mm or 1.706m.

#  International System of Units

The term ‘metric system’ has been superseded by the ‘International System of Units (SI)’ and the units that are used for measuring, such as kilograms and metres, are called SI Units.

There are seven SI base units that form the foundation of the metric system:

 Quantity measured Unit Symbol Length metre m Mass kilogram kg Time second s Electrical current ampere A Thermodynamic temperature kelvin K Amount of substance mole mol Luminous intensity candela cd

From the seven SI base units in the table above, other quantities can be derived (derived quantities) by using quantity equations to cover other measurement requirements:

#  Derived units

 Entity SI derived unit Symbol Area square metres m2 Volume cubic metres m3 Speed, velocity metres per second m/s Mass density kilogram per cubic metre kg/m3 Specific volume cubic metre per kilogram m3/kg Luminance candela per square metre cd/m2 Current density ampere per metre A/m Dynamic temperature degree Celsius °C

Some SI derived units have been given special names and symbols such as watt (W) to represent power and radiant flux; joule (J) to represent energy, work and quantity of heat, and newton (N) to represent force.

Traditionally, metric conversion charts and tables would have been used to convert between different units in the metric system. For example, a table for length and area measurements may show the formulae for how to convert from square kilometres to square metres, or from square metres to square millimetres. Similarly, conversion tables or formulae might be used to convert from imperial units to metric units. More recently, the advent of the internet has ushered in a host of on-line calculators that allow instant and highly accurate conversions once the value to be converted is inputted.

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