The Correlation of Units
The units for different types of measures -- such as length and volume -- correlate with one another. For example, 1 milliliter (a unit of volume) has the same proportions as 1 cubic centimeter (centimeters being a measure of length). A liter of water, by extension, is the equivalent of 1 cubic decimeter. It also has a mass approximately the same as 1 kilogram. This even works by expansion; 1 cubic meter can hold approximately 1,000 liters of water and has about the same mass as 1 metric ton -- or 1,000 kilograms.
And apart from the seven base units in the metric system -- the meter, the kilogram, the second, the ampere, the kelvin, the mole and the candela -- other meaningful units of measurement can be derived from those. Consider velocity. It's typically denoted by a unit of length (kilometers) divided by a unit of time (hours).