As far as I know, 'gram' is used for 1g(one gram), and 'grams' is used for more than

[middleage S]

As far as I know, 'gram' is used for 1g(one gram), and 'grams' is used for more than 2g(two grams). Then, in the case of 0.5g, what do I have to use?

 

[tedmc]

It is like saying "half (a) gram". Less than one is singular.

 

[Tdol]

If you use nought-point-decimal, use the plural. It might seem counter-intuitive, but we can use the plural for numbers below 1. Singular and plural are complex issues in English.

 

[GoesStation]

Zero is also plural.