Dimensionless ratio of the actual fluid density \rho to the fluid density of a reference substance \rho_{\rm ref}, both taken at STP:
(1) | SG= \frac{\rho}{\rho_{\rm ref}} |
The value of \rho_{\rm ref} depends on context:
See also
Natural Science / Physics / Fluid density
[ API gravity ][ Gas specific gravity (γg) ]