**One milliliter equals 1 gram only when the density of a liquid is 1 gram per milliliter.** The most common substance that has 1 gram per milliliter is water. The milliliter is a unit of volume that is one-thousandth of a liter, which is the basic metric volumetric unit. A gram is a metric unit of mass equivalent to one-thousandth of a kilogram.

Density is the measure of compactness of a substance, which is calculated as mass per unit of volume. Density units include kilograms per cubic meter, pounds per square foot and kilograms of liquid per liter. Density is also a quantity per unit of length, area or volume, such as the number of trees per acre. One gram is 0.035 ounces, and 1 milliliter is 0.2 teaspoons.

In addition to 1 gram per milliliter, other chemical properties of water include its high-density point at 4 degrees Celsius, a structure of two hydrogen atoms to one oxygen atom, and its use as a heat regulator in many chemical processes. Water serves as a solvent for acids and bases to create hydroxide and hydronium ions in a solution. Ice is less dense than water, and it therefore floats on top of water. The freezing point of water is 32 degrees Fahrenheit, which is 0 degrees Celsius.