Density is a ratio: mass per volume
Density expresses how much mass is packed into a given volume — grams per cubic centimeter, kilograms per cubic meter, pounds per cubic foot, and so on. Converting density between these units means converting both the mass part and the volume part of the ratio at once.
A convenient reference point: water
Water's density, 1 gram per cubic centimeter, converts to exactly 1,000 kilograms per cubic meter — a clean round number that makes water a useful reference point for comparing other materials.
A denser material, aluminum
Aluminum's density, 2.7 g/cm³, converts to 2,700 kg/m³ — 2.7 times water's density, preserved exactly across the conversion since both figures scale by the same g/cm³-to-kg/m³ factor.
Converting to a mixed-unit system entirely
Converting 1,000 kg/m³ (water's density) to pounds per cubic foot gives 62.427961 lb/ft³ — a less intuitive-looking number because it involves both a mass conversion (kilograms to pounds, from the companion article) and a volume conversion (cubic meters to cubic feet) combined into one multiplication, rather than either conversion alone.