Density Definition: How Tightly Things Are Packed

The science density definition, step by step

Every object has two simple measurements: how much matter it contains (its mass, a measure of matter) and how much space it fills (its volume). Density links them:

Density = mass divided by volume

Scientists often write this with the Greek letter rho. A material with high density packs a lot of mass into a small space, like lead or gold. A material with low density spreads a little mass over a large space, like cork, foam or air. For a pure substance at a given temperature and pressure, density stays the same no matter how big or small the sample is, which is why it helps identify materials.

A worked example

Say you have a small metal block. On a kitchen scale it weighs 270 grams. You drop it into a measuring jug and the water level rises by 100 millilitres, so its volume is 100 cubic centimetres.

1. Mass = 270 g
2. Volume = 100 cubic cm
3. Density = 270 / 100 = 2.7 g per cubic cm

That figure is close to the known density of aluminium, so the block is probably aluminium. You can turn the formula around too: mass = density times volume, and volume = mass divided by density.

Units and some typical values

The SI unit is kilograms per cubic metre; one gram per cubic centimetre equals 1,000 kilograms per cubic metre. Because water sits near 1, anything above 1 g per cubic cm tends to sink in it and anything below tends to float. Ice floats because water expands as it freezes, which makes ice less dense than liquid water.

Other meanings of density

1. Population density. The number of people living in a unit of area, usually per square kilometre or square mile. Large cities in India and the US have very high population densities in their central districts.
2. Thickness or closeness. The density of a forest, of fog, of hair or of a fabric's weave.
3. Bone density. A medical measure of how much mineral is in bone, used to check bone strength.
4. Information density. How much meaning or data is packed into text, a chart or a storage device. A dense textbook chapter takes slow reading.

Example sentences

• Oil floats on soup because its density is lower than that of water.
• The planners want to raise housing density near the new metro stations.
• Her doctor ordered a scan to check her bone density.
• The density of the fog made the morning flights late.
• This report has a high density of numbers, so add a summary at the top.

Dense, density and mistakes to avoid

Dense is the adjective and density the noun: "a dense crowd" but "the density of the crowd." Do not confuse density with weight. A tonne of feathers and a tonne of steel weigh the same, but the steel has a much greater density, so it fills far less space. Also, density is not the same as hardness: some dense materials, like lead, are soft. In casual speech, calling a person "dense" means slow to understand, which is an insult; the noun density is not normally used that way. For more science vocabulary, see inertia, osmosis and our science and ideas hub.

Meaning of density through history

Density came into English around 1600 from French densité and Latin densitas, thickness, built on densus, thick or crowded. At first it described anything close or compact. The physics sense of mass per unit of volume is recorded from the later 1600s, as natural philosophers began measuring materials more precisely.