Density Converter – kg/m³, g/cm³ and lb/ft³

What density measures

Density is mass per unit of volume: density = mass ÷ volume. The SI unit is the kilogram per cubic metre (kg/m³). The converter uses these factors:

• 1 g/cm³ = 1,000 kg/m³
• 1 g/ml = 1 g/cm³ = 1,000 kg/m³ (a millilitre is a cubic centimetre)
• 1 lb/ft³ = 16.0185 kg/m³
• 1 lb/gal (US) = 119.826 kg/m³

The numbers in g/cm³ and kg/L are identical, and multiplying g/cm³ by 1,000 gives kg/m³.

Typical densities

• Air at sea level: about 1.2 kg/m³
• Petrol: about 740 kg/m³ (0.74 g/cm³)
• Water: about 1,000 kg/m³ (1 g/cm³ or 62.4 lb/ft³) at 4 °C
• Concrete: about 2,400 kg/m³
• Aluminium: 2,700 kg/m³
• Steel: about 7,850 kg/m³ (490 lb/ft³)
• Gold: 19,300 kg/m³

Materials with a density lower than water, such as most wood and oil, float on it.

Using density in calculations

If you know the density and the volume, the mass is density × volume. A steel plate of 0.01 m³ weighs 0.01 × 7,850 = 78.5 kg. Building suppliers use this to estimate the weight of concrete, gravel and sand per cubic metre; the concrete and gravel calculators work the same way.

Specific gravity

Specific gravity (relative density) is the density of a substance divided by the density of water. Because water is about 1 g/cm³, specific gravity has almost the same number as the density in g/cm³. It is common in brewing, battery testing and gemology. Water weighs about 8.34 pounds per US gallon, which is why that figure appears in US engineering.

Temperature matters

Most liquids and gases expand when heated, so their density drops. Fuel is delivered at a reference temperature, and laboratories state the temperature with every density measurement.

Sources

Conversion factors follow the SI definitions in the BIPM SI Brochure and the exact factors for US customary and imperial units in NIST Special Publication 811.