The density formula (ρ = m / V)
Density is defined as mass per unit volume: ρ = m / V. In SI units, density is measured in kilograms per cubic metre (kg/m³) or grams per cubic centimetre (g/cm³), where 1 g/cm³ = 1,000 kg/m³.
In US customary and British imperial units, density is commonly expressed in pounds per cubic foot (lb/ft³), where 1 g/cm³ ≈ 62.428 lb/ft³.
Specific gravity (SG), or relative density, is the dimensionless ratio of a substance’s density to the density of pure water at 4°C (1,000 kg/m³). Substances with SG < 1 float in water, while those with SG > 1 sink.
Worked example: Density of structural steel
A structural steel beam has a mass of 785 kg and a volume of 0.10 cubic metres (100 litres).
Density = 785 kg ÷ 0.10 m³ = 7,850 kg/m³ (7.85 g/cm³ or 489.96 lb/ft³).
Its specific gravity is 7.85, confirming that steel is 7.85 times denser than water.
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CALCULATOR QUESTIONS
Density, mass & volume calculator questions
What is the density of pure water?
At 4°C, pure water has a density of 1,000 kg/m³, 1.0 g/cm³, 1.0 kg/L, or approximately 62.43 lb/ft³.
What is specific gravity?
Specific gravity is the ratio of a material density to the density of water. It has no units. A specific gravity greater than 1 means the object will sink in water.