Rule1- Lower case is used to write the names of units.Units named after the scientists too are written in lower case,for example,newton,metre,kelvin etc.
rule2- Symbols of units that bear the name of the scientists are written in upper case.
rule3- Other symbols not named after scientists are written in lower case.
Example- symbol for second- s
symbol for metre- m
symbol for kilogram- kg
symbol for gram- g
rule4- Symbols of units are never followed by a full stop.
Symbol for kilogram is kg (not kg.)
Symbol for centimetre is cm (not cm.)
However, when a sentence ends with the symbol of a unit,it is followed by a fullstop.
rule5- Units are written in plural only when used in words, but symbols remain unaltered.
symbols for 5 metres is 5m (not 5ms)
symbols for 3 kilograms is 3 kg (not 3 kgs)