# Density

How much mass is packed into each unit of volume. Density = mass / volume, in kg per cubic metre (kg/m3).

Physics (school) · Part 04: Particles and heat · Card 1 of 8 · Last updated 2026-10-19

## Analogy

A bag of feathers and a bag of sand are the same size, but sand has far more stuff packed in.

## Example

**Block of metal**

mass = 240 kg
volume = 0.03 m3
density = 240 / 0.03
= 8,000 kg/m3

## In practice

Objects float when they are less dense than the liquid they sit in.

## Practice

### Flashcards

- **What is the formula for density?** Density = mass / volume.

## Questions people ask

### Why does a steel ship float?

The ship is hollow and full of air, so its overall density, with the air included, is less than the density of water. A solid lump of steel sinks because steel is denser than water. The shape of a hull spreads the steel over a big volume.

### How do you measure the density of an irregular object?

Find its mass using a balance. Find its volume by lowering it into a measuring cylinder of water and noting how far the level rises. Then divide mass by volume. A stone of 90 g that raises the water by 30 cm3 has a density of 3 g/cm3.

## Related concepts

- [States of matter](https://studyvisually.com/learn/science/physics-school/physics-school-part-4/physics-school-part-4-concept-2)
- [Changes of state](https://studyvisually.com/learn/science/physics-school/physics-school-part-4/physics-school-part-4-concept-3)
- [Gas pressure](https://studyvisually.com/learn/science/physics-school/physics-school-part-4/physics-school-part-4-concept-8)

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