Length is one of the most fundamental quantities in the metric system. It is used to describe how long something is or how large the distance is between two points. The unit of length in the SI system is the metre (m).

 

 

Common length units

Today, the metre is used as the standard unit, but there are many practical subunits and multiples that are easier to use depending on the situation.

 

Unit	Symbol	Relation
millimetre	mm	\( \large 1\ \text{mm} = 0.001\ \text{m} \)
centimetre	cm	\( \large 1\ \text{cm} = 0.01\ \text{m} \)
decimetre	dm	\( \large 1\ \text{dm} = 0.1\ \text{m} \)
metre	m	\( \large 1\ \text{m} = 1\ \text{m} \)
kilometre	km	\( \large 1\ \text{km} = 1000\ \text{m} \)

 

 

Conversion between length units

Because the metric system is based on powers of ten, it is very easy to convert between different length units. You simply move the decimal point according to how many powers of ten the difference represents.

 

For example, you can convert from kilometres to metres by multiplying by 1000, and from metres to centimetres by multiplying by 100.

 

$$ \large 1\ \text{km} = 1000\ \text{m} $$

$$ \large 1\ \text{m} = 100\ \text{cm} $$

 

 

Example

A school route is 2.4 kilometres long. How many metres is that equivalent to?

 

$$ \large 2.4\ \text{km} = 2.4 \cdot 1000 = 2400\ \text{m} $$

 

The school route is therefore 2400 metres long.

 

The metric system makes it possible to convert between units without remembering complicated conversion factors — you simply move the decimal point by a multiple of ten.

 

 

Summary

When converting length units, you should:

 

• remember that each unit differs by a factor of 10
• move the decimal point one place for each step up or down the scale
• use the same prefixes (milli-, centi-, deca-, kilo-) as in other SI units

 

In this way, length forms the basis for the derived quantities area and volume, which are simply the length unit raised to the second and third power respectively.