Unit symbols are always singular (ie they are not followed by s) and in running text are treated as singular:
10 mL was added. not 10 mL were added.
During the year, $20 million was donated. not During the year, $20 million were donated.
If the name of the unit is written out, the plural form takes an s:
0.5 kilogram 1 gram 18 millimetres 7 litres 2 kilometres 1 degree Celsius 2 degrees Celsius
but it is treated as singular in running text:
In all, 10 millilitres was added.