What is Density?

Instead of going straight to the formula, I will talk about the concept of volumetric density (Summary).

Many think that density is just ratio mass / volume of material, but this concept encompasses more than that and includes others, such as compression (Physics) and the packaging of atoms (Chemistry).

The greater the packing of atoms, the more density the material will have. Similarly, the more compression this material undergoes, the greater its density.

After explaining volumetric density, I will talk about the linear or superficial densities that are used in our day to day.

Definition of Volumetric Density

There are two types of volumetric density:
  • Absolute
  • Relative

Absolute Density

Density (ρ) is a relationship between mass (m) and its volume (V). Look at the formula below:

ρ = m / V

Absolute density is a specific property. In another words, each pure substance has a unique density, which differentiates it from other materials.


Relative density is given by the formula below:

d = ρ / ρ0

It is the relation between its absolute density and that of another material established as the standard.

The standard usually chosen in this calculation is the absolute density of water, which is approximately equal to 1 kg dm⁻³ (equivalent to 1,000 g cm⁻³) at 4 ºC.


The density of impure materials depends on their composition. This is the case of steel where small additions of alloying elements slightly affect the density of the "steel" material.

Density is an important property and can be used to differentiate a pure material from an impure (or alloys of that metal as is the case with steel). Therefore it can be used in the quality control of an industrial product.

Determining density

Density of a solid material is determined by weighing and then determining its volume.

When the solid presents irregular shape (practical impossibility of measuring its dimensions), the volume can be determined by the displacement method.
Densidade do Aço

Displacement Method

This method consists of transferring the solid material to a graduated volumetric instrument (eg, beaker), partially filled with water or other dense liquid. The solid will displace a volume of liquid equal to its own volume.
Determinando a densidade

We take note of the volume before and after the addition of the material we wish to measure. Then we deduce the two values to determine the volume of our material.

Linear and Surface

Linear Density

Density is the ratio of mass to length (line).

Example: 750g and 1000m length cable.

The linear density is calculated as follows:

Surface Density

Surface Density is the ratio mass to surface area.

Example: 500 kg sheet, 2 m wide and 4 m long.

The surface density is calculated as follows:

Real World Examples

Some examples:


Who is Gelson Luz anyway?

Gelson Luz is a Mechanical Engineer, expert in welding and passionate about materials. Search for "Gelson Luz" in your favorite browser to learn more.

Materials: Density
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