Is 18000 g greater than 9000 mm?

Answer 1

The question as asked is nonsense: you are trying to compare the measurements of two different aspects of things:

• the gram (g) is a measure of mass; whereas
• the metre (m - in this case prefixed by milli- to make mm) is a measure of length.

Answer 2

There can be no comparison. A gram (g) is a measure of mass. A millimetre (mm) is a measure of length. The two measure different things and, according to basic principles of dimensional analysis, conversion from one to the other is not valid.