Usually specific density is measured relative to the density of water.
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The difference between density and specific gravity is that one is a ratio of the other.
Density is the measure of the amount of mass per volume.
Density is expressed by the formula
ρ = m/V where
ρ is the density
m is the mass
V is the volume
Specific gravity is a measure of density relative to the density of a reference substance.
Specific gravity is also called relative density and is expressed by the formula:
Specific Gravitysubstance = ρsubstance/ρreference
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.