Density is mass divided by volume (ρ = m / V). The SI unit is kilograms per cubic meter (kg/m³), but g/cm³, lb/ft³ and other unit pairs are common depending on the field.

A familiar reference point: water is about 1000 kg/m³, which is exactly 1 g/cm³ (or 1 g/mL) — a handy benchmark for judging whether a material floats or sinks.