What Is the Difference Between Density and Specific Gravity?

Density vs Specific Gravity

Usually specific density is measured relative to the density of water.
Usually specific density is measured relative to the density of water. Howard Shooter / Getty Images

The difference between density and specific gravity is that specific gravity is an expression of density in relation to the density of a standard or reference (usually water). Also, density has units, while specific gravity is a pure number or dimensionless. Both density and specific gravity describe mass and may be used to compare different substances.

What Is Density?

Density is the measure of the amount of mass per volume.

It's typically expressed in units of grams per cubic centimeter, kilograms per cubic meter, or pounds per cubic inch.

Density is expressed by the formula

ρ = m/V where

ρ is the density
m is the mass
V is the volume

What Is Specific Gravity?

Specific gravity is a measure of density relative to the density of a reference substance. It's often abbreviated as sp gr.

Specific gravity is also called relative density and is expressed by the formula:

Specific Gravitysubstance = ρsubstancereference

The reference material could be anything, but the most common reference is pure water. One example is salt water aquarium enthusiasts measure the amount of salt in their water by specific gravity where their reference material is fresh water. The units cancel each other out, so specific gravity is just a number. If a material has a specific gravity less than 1, it will float on water.

Converting Between Density and Specific Gravity

Specific gravity values aren't very useful except for predicting whether or not something will float on water and for comparing whether one material is more or less dense than another.

However, because the density of pure water is so close to 1 (0.9976 grams per cubic centimeter), specific gravity and density are nearly the same value so long as the density is given in g/cc. Density is very slightly less than specific gravity.