Despite efforts to 'turn metric', it is obvious that there are still many imperial units in use today. For example do you know your height in centimetres and your weight in stones and pounds? Do you cycle for 5 miles but run 100 metres? It is therefore helpful to know how to convert between the two.
Perhaps the most common metric conversions are between mm, cm, m and km.
1 cm = 10 mm
1 m = 100 cm = 1000 mm
1 km = 1000 m = 100 000 cm = 1 000 000 mm
kilo means 1000, so 1 km = 1000 m
centi means 1/100th, so 1 cm = 1/100th m
milli means 1/1000th, so 1 mm = 1/1000th m
Once you are confident with conversions between mm, cm, m and km, converting units of mass and capacity is easier.
1 km = 1000 m, so 1 kg = 1000 g
1 m = 100 cm, so 1 l (litre) = 100 cl (centilitres)
1 m = 1000 mm, so 1 g = 1000 mg (milligrams)