Introduction
Density — mass per unit volume — is the fingerprint of a material. Water is ~1,000 kg/m³ at room temperature, steel ~7,850, air ~1.225, and human body fat ~900 (which is why body fat floats). Converting between kg/m³, g/cm³, and lb/ft³ is the constant bridge between SI and US engineering work.
Why density units exist and how they diverged
Density couples mass to volume, so its conversion factors are combinations of mass and volume factors. 1 g/cm³ = 1,000 kg/m³ (the gram and the cm³ happen to align so water has density 1 in both g/cm³ and kg/L). 1 lb/ft³ = 16.02 kg/m³. Water is the accidental reference point — its density is 1.000 g/cm³ at 4°C (by the 1795 definition of the kilogram), which is why specific gravity (density relative to water) is a common shorthand.
Specific gravity = density / 1,000 kg/m³. Ice is 0.917; seawater 1.025; blood 1.060; mercury 13.534. Any specific gravity < 1 floats on water; > 1 sinks.
How to convert density
g/cm³ to kg/m³: multiply by 1,000. lb/ft³ to kg/m³: multiply by 16.02. lb/gal (US) to kg/m³: multiply by 119.83. For weight calculations: mass (kg) = density (kg/m³) × volume (m³). A 1-m³ cube of concrete (density ~2,400 kg/m³) weighs 2,400 kg.
Units supported by this density calculator
- kg/m³
- g/cm³
- g/mL
- kg/L
- lb/ft³
- lb/in³
- lb/gal (US)
- lb/gal (UK)
- oz/in³
- slug/ft³
Common density conversion mistakes
- Specific gravity as a unit. Specific gravity is dimensionless; density has units. 'Density 1.5' is incomplete; specify g/cm³ or kg/m³ or specific gravity.
- Temperature effects. Water density drops from 1,000 kg/m³ at 4°C to 958 kg/m³ at 100°C. For precision work always include temperature.
- API gravity (petroleum). An inverse-density scale used in oil industry. API = 141.5/SG - 131.5. Light oil has API 30+; heavy crude has API 20-.
- lb/gal US vs UK. 1 US gallon (3.785 L) vs 1 UK gallon (4.546 L). A 'pound per gallon' conversion factor depends on which gallon.
- Pressure effects. Gases compress; liquids mostly don't. Air density at sea level is 1.225 kg/m³; at 10,000 ft it's 0.905 (26% less). Water density at 1,000 m depth rises only 0.5%.
Real-world density examples
- Air (sea level, 15°C): 1.225 kg/m³.
- Water (20°C): 998 kg/m³.
- Seawater: 1,025 kg/m³.
- Ice: 917 kg/m³ (why ice floats).
- Gasoline: 740 kg/m³.
- Ethanol: 789 kg/m³.
- Milk: 1,030 kg/m³.
- Honey: 1,420 kg/m³ (sinks in water).
- Concrete (cured): 2,400 kg/m³.
- Aluminum: 2,700 kg/m³.
- Steel: 7,850 kg/m³.
- Lead: 11,340 kg/m³.
- Gold: 19,320 kg/m³.
- Mercury (liquid metal): 13,534 kg/m³.
- Osmium (densest natural element): 22,590 kg/m³.
- Neutron star core: ~4 × 10¹⁷ kg/m³.
Tips for accurate density conversion
- For chemistry, g/cm³ or g/mL is standard. Water is 1.00 at 4°C.
- For civil/mechanical engineering, kg/m³. Concrete, soil, steel.
- For fluids at temperature, always cite T — density varies.
Related: Weight Converter · Volume Converter · Pressure Converter.