Singular or plural units

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.

Australian manual of style