The specific gravity or a substance is always numerically equal to its mass density

Explanation

When using water as the reference substance:

• The density of water is 1 g/cm³ (or 1000 kg/m³) in common units

Given this reference:

• If a substance has a density of 2 g/cm³, its specific gravity relative to water is:

$SG = \frac{2\text{ g/} \text{cm}^3}{1 \text{ g/}\text{cm}^3}$

So, numerically, the specific gravity of a substance is equal to its mass density in units where the density of water is 1.