The problem with this type of conversion is that it isn't as simple as it sounds because the units involved are different. A gram is a unit of weight and ml is a unit of volume.
To complete a calculation such as this, you need to be able to factor in the density of the substance that you are looking to convert and include that in the formula calculation.
For substance with density: 1602 g/cm3 like sand (dry) (rounded to 3 decimal places), 30 g is equivalent to 0.019 ml.
Use this Weight To Volume Converter to help convert between weight and volume.
Find out How to Convert Units of Measurement from BrownMath.com.