Converting between metric and imperial units
In a nutshell
The imperial unit system is not used as frequently as the metric unit system but it is still used for some common measurements. The metric and imperial systems can represent the same measurement in a different way and you must be able to convert between the imperial and metric systems.
Metric and imperial units
The metric and imperial system use different units. It is useful to know the approximate values of imperial units in the metric system.
LENGTH  $1 \space inch \approx 2.5cm$  $1\space foot \approx30cm$  $1 \space mile \approx 1.6km$ 
MASS  $1 \space ounce \approx 30g$  $1 \space pound \approx 0.5kg$  $1 \space stone \approx 6.5kg$ 
VOLUME/CAPACITY
 $1 \space pint \approx 0.5 \space litres$  $1 \ gallon \approx 4.5 \ litres$

Note: You do not need to memorise these conversions as they will be stated in the question.
Example 1
Adam's height is $5$ foot $6$ inches. Use the conversions from the table above to calculate his height in centimetres.
To convert from feet to centimetres multiply by $30$.
$5 \ \times 30 = 150cm$
To convert from inches to centimetres multiply by $2.5.$
$6\ \times 2.5 = 15cm$
$150cm + 15cm = 165cm$
Therefore, Adam is approximately $\underline{165cm}$.
Example 2
How many kilometres are there in $5$ miles? Use the conversions from the table above.
$1 \ mile \approx 1.6km$
To convert miles to kilometres multiply by $1.6$.
$5 \times 1.6 = 8km$
Therefore, $\underline{ 5 \ miles \approx 8km}$.