Hello April.

To convert a liquid's volume (milliliters) to its mass (grams), you multiply by the density of the liquid. Sometimes density is called "specific gravity" for obscure reasons.

Milk is mostly water, so you can approximate the answer by simply noting that for pure water, 1 ml = 1 g because the density of water is 1.0000000 g/ml.

If you need more accurate measurement, then it will depend on the milk itself. How much fat is in it? What is the temperature? How many "milk solids" are in it? All these things affect the density of milk. Here is a link to a web page with a formula to use if you need this level of accuracy:
classes.aces.uiuc.edu/AnSci308/Milkcompsynth/milkcomp_density.html

Hope this helps,
Stephen La Rocque.