Density is mass per unit volume (ρ = m/V). The SI unit is kilograms per cubic metre (kg/m³), but grams per cubic centimetre (g/cm³) is more practical for laboratory work — and numerically equal to g/mL and kg/L, since 1 cm³ = 1 mL and 1 L = 1 dm³. Water has a density of approximately 1 g/cm³ (1000 kg/m³) at 4 °C, making it a convenient reference point.
Imperial units use pounds per cubic foot (lb/ft³) for construction materials and lb/in³ for metals and machined parts. Density is critical in buoyancy calculations (Archimedes' principle), material selection, shipping weight estimates, and mixing ratios in chemistry and cooking.
| Water (4°C) | 1.000 g/cm³ |
| Aluminum | 2.70 g/cm³ |
| Iron | 7.87 g/cm³ |
| Gold | 19.3 g/cm³ |
| Air (sea level) | 1.225 kg/m³ |
| Oak wood | ~700 kg/m³ |