Anton,

Since grams are a measurement of mass, and "mils" or millilitres are a measurement of volume, the conversion between them is as follows.

mass = volume * density

where mass is in units of grams, volume is in units of millilitres (or cubic centimetres), and density is in units of grams per millilitre (or cubic centimetre).

Therefore, the conversion of a certain mass to a corresponding volume is dependent on the density of the substance involved. In the case of weight loss powder, you probably don't know the appropriate density, since the powder will be made of crystals and an uncompressed pile of the crystals will have some spaces in between the crystals. The best way to determine the "effective" density would be to use a scale to determine the mass of a known volume of the powder.

I hope this helps. It may also interest you to know that water is a special case where

1 millilitre = 1 gram

therefore the density of water is 1 gram per millilitre. This is for water at 4°C, since density changes with temperature.

Many common substances, like milk and the human body, have similar densities to water, since they are mostly made of water. However, your powder probably has a considerably different density from this.

Finally, you may want to look somewhere (like nutritional information) on the package for a quote from the manufacturer which lists a certain mass and then in brackets a certain volume or vice versa. This would allow you to calculate the effective density without actually measuring anything. For example, my bottle of whey powder says in the nutritional information box "per 2 scoops (1/4 cup, 35 g)", and since i know that 1/4 cup is 62.5 mL, (1 cup = 250 mL)

density = 35 g / 62.5 mL = 0.56 g/mL.

Your powder is probably similar. Hope this helps!
Gabriel