Density and Specific Gravity; Difference and Explanations

In the lots of calculations, we are using density and the specific gravity properties of masses and substances. But we need to know the difference between gravity and density. Here, we explain the difference between the densities and specific gravities of substances.

English units vs SI units. You can see the difference between specific gravity and density here.
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What is the Difference between Density and Specific Gravity?

First of all, we need to define these two important physical units.

What is the Density?

Density is the mass per unit volume. It is an intensive property of materials. So, density is an independent value of mass. So with the changing mass, density does not change. The unit of density is lbs/ft^3 or kg/m^3 in the SI units. For example, the density of gold is 1206 lb/ft^3. We can understand from this value that, the 1 ft^3 of pure gold element weighs 1206 lbs which is very heavy compared with other metals. 

As you understand, we can compare the masses of different substances by using their density knowledge of them.

What is the Specific Gravity?

The specific gravity is the ratio of the density of a substance to another reference substance which is generally water. At a specific pressure and temperature, the specific gravity of a reference substance is defined. For example in the SI units, the density of water at 4°C is the reference density. The density of the water is 62.4 lbs/ft3 or 1000 kg/m^3. According to this reference, the specific gravities of other substances are;

  • Mercury: 13,6,
  • Gold: 19.3.

As you see above, the specific gravity is a unitless value. So, this unitless system gives a very good comparison between the substances.

Difference between Them

In SI units, the density of water is 1000 kg/m^3. Because of it, the specific gravities of substances in SI units are the same as the densities. Because the reference is 1 and we divide thist by 1 to find specific gravity.

But in the English units, the density of water is 62.4 lbs/ft3 which is not 1. So the specific gravities of substances are not equal to their densities in English units.


The general confusion between the specific gravity and the density comes from this situation in SI units. They are appearing same and people understand these values are the same. But they are not.

The general explanation of the difference between specific gravity and density is as above.

Finally, do not forget to leave your comments and questions below about the specific gravity and density values of substances.

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