What is Specific Gravity?

• A. The ratio of density of a fluid to the density of a reference fluid
• B. The weight per unit volume of a fluid
• C. The vapor pressure of a liquid
• D. The boiling point of a fluid

Answer: (A)

Specific gravity is a comparison of how dense a fluid is to the density of a reference fluid, giving a dimensionless number. It is defined as the density of the fluid divided by the density of the reference fluid. For liquids, the reference is usually water at 4°C, so the value tells you how many times denser the liquid is than water at that temperature. Because both densities cancel their units, no units remain. This makes specific gravity a convenient way to compare fluids across temperatures and units. For example, a liquid with density 0.95 g/mL has a specific gravity of 0.95 relative to water, while a denser liquid would have a higher SG. In practice, instruments like hydrometers or densitometers use this ratio to identify fluids or monitor mixtures. Other properties—weight per unit volume (specific weight), vapor pressure, and boiling point—describe different characteristics and are not what specific gravity measures.