Specific Gravity, SG

Specific Gravity, SG of a fluid is the ratio of its density to that of water under reference conditions, usually at $4^0 C$ (i.e., $1000 kg/m^3$.)

$${SG} = {{\rho} \over {\rho_{water  at  4^0 C}}}$$ (1.16)

Specific Gravity being a ratio of densities is independent of units. Its value for mercury at $20 ^0C$ is 13.55. There does not seem to a set standard for reference density. Sometimes for gases density of air under standard conditions $1.205 kg/m^3$ is also used as the reference.

(c) Aerospace, Mechanical & Mechatronic Engg. 2005
University of Sydney