Metric Units

Kilograms per Cubic Meter (kg/m³): The standard SI unit of density. Water at 4°C has a density of approximately 1000 kg/m³.

Grams per Cubic Centimeter (g/cm³): A common laboratory unit. Numerically equal to specific gravity in reference to water. 1 g/cm³ = 1000 kg/m³.

Grams per Milliliter (g/mL): Used in chemistry and is equivalent to g/cm³ since 1 mL = 1 cm³.

Grams per Liter (g/L): Used for expressing concentrations in solutions. 1 g/L = 1 kg/m³.

Imperial/US Units

Pounds per Cubic Foot (lb/ft³): Common in engineering in the United States. Water is approximately 62.4 lb/ft³.

Pounds per Cubic Inch (lb/in³): Used for dense materials in US engineering.

Pounds per Gallon (lb/gal): Used in the US for liquid densities, especially in the petroleum industry.

Slugs per Cubic Foot (slug/ft³): Used in US engineering analysis. A slug is a unit of mass equal to 32.174 pounds.

Temperature Dependence

The density of most materials changes with temperature. For most substances, density decreases as temperature increases (because volume increases faster than mass). Water is an important exception between 0°C and 4°C, where its density actually increases with temperature.

Relative Density or Specific Gravity

Relative density (or specific gravity) is the ratio of the density of a substance to the density of a reference substance, typically water for liquids and solids, and air for gases. Being a ratio, it is dimensionless and has no units.

Applications of Density

• Buoyancy: Objects float in fluids that are more dense than they are.
• Material Identification: Density can help identify unknown materials.
• Quality Control: Changes in density can indicate changes in composition or manufacturing processes.
• Oceanography: Density differences drive ocean currents and layering.