What is one millilitre to one gram?

Answer 1

Unfortunately, a milliliter is a unit of volume, and a gram is a unit of weight, so there isn't a way to convert them. Luckily, if you are referring to pure water, the volume of one gram of water is exactly one milliliter.

This only works for water, though.

Answer 2

For all practical purposes an ml and a gm are equal provided that the material is a liquid which is similar to water. As an example the density of molasses is 1.45 times as high as water and 1 ml would be alnost 1.5 gm (or 1 gm would be 0.8 ml). Thi doesn't sound significant for one gram amounts, but measurements often involve 500 ml or more

This becomes even more problematic if dry materials are being measured using a cup measure marked in ml as well as cups and a mass recipie is being followed as happens in many British cookbooks.

There is no consistent accurate relation between a millilitre (volume) and a gram (mass). Special circumstances can be arrived at by adjusting the observation conditions (such as water at 4oC) where there is an equivalency.