Hi Larissa.

Grams measure mass (weight) and milliliters measure volume, so you can't "convert" one to the other. Most often however, there is a mass of a particular substance that you need and you want to find how many milliliters of it that corresponds to. For that, you need to know the density (or specific gravity) of the substance.

For example, the density of olive oil is 0.9 g/ml so 10 g of olive oil would fill 11.11 ml in a container. Compare that to iodine which is 4.93 g/ml. 10 g of iodine would correspond to just 2 ml. Pure water at 4C (yes temperature also affects the density) is 1.0000 g/ml.

Hope this helps,
Stephen La Rocque.