Both density and specific gravity describe mass and may be used to compare different substances. They are not, however, identical measures. Specific gravity is an expression of density in relation to the density of a standard or reference (usually water). Also, density is expressed in units (weight relative to size) while specific gravity is a pure number or dimensionless.
What Is Density?
Density is a property of matter and can be defined as the ratio of mass to a unit volume of matter. 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. The reference material could be anything, but the most common reference is pure water. If a material has a specific gravity less than 1, it will float on water.
Specific gravity is often abbreviated as sp gr. Specific gravity is also called relative density and is expressed by the formula:
Specific Gravitysubstance = ρsubstance/ρreference
Why would someone want to compare the density of a substance to the density of water? Take this example: Saltwater aquarium enthusiasts measure the amount of salt in their water by specific gravity, where their reference material is freshwater. Saltwater is less dense than pure water but by how much? The number generated by a calculation of specific gravity provides the answer.
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.