# Specific gravity

Specific Gravity (SG) is a special case of relative density defined as the ratio of the density of a given substance, to the density of water (H2O). Substances with a specific gravity greater than 1 are heavier than water, and those with a specific gravity of less than 1 are lighter than water.

${\displaystyle {\mbox{SG}}={\frac {\rho _{\mathrm {substance} }}{\rho _{\mathrm {H} _{2}\mathrm {O} }}}}$

The density of the denominator, or reference, is ${\displaystyle \rho _{\mathrm {H2O} }}$ = 1000kgm-3 (at 4°C/39.2°F) in SI units.

SG is by definition dimensionless and therefore not dependent on the system of units used (e.g. slugsft-3 or kgm-3), in so far as the units are consistent. Based on the SG-value of a given substance, the density of that substance can be calculated.