There is exactly 1 milliliter of liquid in a gram. This is due to the conversion rate created by the International System of Units, which sets rate for volumes by the cubic meter, each of which is 1,000,000 millimeters or 1,000,000 grams.
The milliliter is a measurement of volume that is accepted by the International System of Units, though it is not a standard measurement in that system. A single milliliter is one-thousandth of a liter, which is convertible to one cubic meter, where each edge of a cube measures a meter in length. These measurements are based on water conversions.