EngineeringFormulaDensity (ρ)
Density is mass per unit volume - how much a given volume of a substance weighs.
It's the proportionality constant between pressure and depth in hydrostatics, between buoyant force and displaced volume in Archimedes' principle, and between inertial and viscous forces in the Reynolds number. Base SI unit is kg/m³; US customary work commonly uses lb/ft³.
Density is what determines whether an object floats: an object floats if its average density is less than the fluid it's placed in, regardless of its total mass or size - a steel ship floats because its hull encloses enough air to bring its average density below water's, not because steel itself is buoyant.
For calculator inputs, use the actual fluid or material density at the relevant temperature when possible. Water at room temperature is close to 1,000 kg/m³, but oils, air, seawater, concrete, and metals vary enough that a default can create a misleading result.
Typical Densities by Material
| Material | kg/m³ | lb/ft³ |
|---|---|---|
| Air (sea level, 15°C) | 1.225 | 0.0765 |
| Gasoline | 720–770 | 45–48 |
| Fresh water (4°C) | 1,000 | 62.4 |
| Seawater | ~1,025 | ~64 |
| Aluminum | 2,700 | 168.6 |
| Structural steel | 7,850 | 490 |
Used In These Calculators
Related Terms
Frequently Asked Questions
Is density the same as specific gravity?
Related, not identical. Specific gravity is a substance's density divided by water's density - a dimensionless ratio (e.g. a specific gravity of 0.8 means 800 kg/m³). This site's calculators take density directly, in real units, rather than specific gravity.
Why does a steel ship float if steel is denser than water?
Buoyancy depends on the object's average density, not the density of the material it's made from. A ship's hull encloses a large volume of air, so the ship as a whole - hull plus enclosed air - displaces enough water to have an average density below water's, even though the steel itself (about 7,850 kg/m³) is roughly eight times denser than water.
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