To convert millimeters to grams, you need to know the density of the material you are measuring. Different materials have different densities, which means they have different mass per unit volume. For example, water has a density of 1 gram per cubic millimeter, while gold has a density of 19.3 grams per cubic millimeter.
If you are measuring water, then 8 mm³ = 8 mg = 0.008 g. If you are measuring gold, then 8 mm³ = 8 x 19.3 mg = 154.4 mg = 0.1544 g. You can use the unit converter to find the density of other materials and convert them to grams.
I hope this helps you understand how to convert millimeters to grams.