Specific gravity is a dimensionless quantity, meaning it doesn't have units. It's calculated as the ratio of the density of a substance to the density of a reference substance.

Here's the formula:

Specific Gravity = Density of Substance / Density of Reference Substance

Commonly, the reference substance is water at 4°C.

Important Note: This formula assumes that the density of the reference substance is known. If the density of the reference substance is not known, then you can't calculate the specific gravity.