Definition · Plain-language
Density
Density is the amount of mass contained in a given volume — how tightly packed the matter in an object is — found by dividing its mass by its volume.
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Mass packed into volume
Density describes how concentrated the matter in an object is — how much mass occupies a given volume. A kilogram of lead and a kilogram of feathers have the same mass, but the lead takes up far less space, so it is much denser. Mathematically, density is mass divided by volume. Because it is a ratio of two properties of the same substance, density does not depend on how much of the substance you have: a small chip of gold and a large bar have exactly the same density. This makes density a useful fingerprint for identifying materials.
The formula and its units
The formula is density equals mass divided by volume, often written ρ = m ÷ V. To find the density of an object you measure its mass, measure or calculate its volume, and divide. The SI unit is the kilogram per cubic metre, but for everyday objects grams per cubic centimetre is more convenient — water, conveniently, has a density very close to 1 gram per cubic centimetre. Rearranging the same formula lets you find mass from density and volume, or volume from mass and density, which is frequently useful in practice.
Why things float or sink
Density explains one of the most familiar everyday phenomena: whether something floats. An object placed in a fluid floats if it is less dense than that fluid and sinks if it is denser. A block of wood floats on water because wood is less dense than water; a coin sinks because metal is denser. This is also why a huge steel ship floats — its hull encloses a large volume of air, making the ship’s overall density less than water’s, even though steel itself is far denser. Temperature matters too, since heating usually lowers a substance’s density.
Key facts
At a glance
- Definition: mass per unit volume — how tightly matter is packed
- Formula: density = mass ÷ volume (ρ = m ÷ V)
- SI unit: kilograms per cubic metre (kg/m³)
- Common unit: grams per cubic centimetre (g/cm³)
- Water: about 1 g/cm³, the reference for floating
- Floating: less dense than the fluid floats; denser sinks
Common misconceptions
What people often get wrong
Often heard: Heavier objects are always denser than lighter ones.
Actually: Not necessarily. Density compares mass to volume, not mass alone. A large, light object can be denser than a small, heavy one if it packs more mass into each unit of space.
Often heard: Density changes if you take a larger piece of the same substance.
Actually: It does not. Density is intrinsic: a small and a large piece of pure gold have the same density, because mass and volume rise together.
Often heard: Heavy materials like steel cannot float.
Actually: A steel ship floats because its overall density — steel hull plus enclosed air — is less than water’s. Shape, not just material, determines whether something floats.
Going deeper
