Specific Gravity, SG of a fluid is the ratio of its density to that of water under reference conditions, usually at (i.e., .)
Specific Gravity being a ratio of densities is independent of units. Its value for mercury at is 13.55. There does not seem to a set standard for reference density. Sometimes for gases density of air under standard conditions is also used as the reference.
(c) Aerospace, Mechanical & Mechatronic Engg. 2005
University of Sydney