What does specific gravity compare?

• A. The ratio of the density of the fluid to the density of air.
• B. The ratio of the density of the fluid to the density of pure water.
• C. The density-to-weight ratio of the fluid.
• D. The viscosity of the fluid relative to water.

Answer: (B)

Specific gravity is the ratio of a fluid’s density to the density of pure water. This ratio is dimensionless, so it lets you compare how heavy a liquid is relative to water without worrying about units. Water is used as the reference (about 1.0 g/cm3 at 4°C), so fluids with the same density have an SG of 1, denser fluids have SG > 1, and lighter fluids have SG < 1. In aviation maintenance, this helps quickly evaluate liquids like fuels, oils, and battery electrolytes. The other options don’t fit: comparing to air is for gases, the density-to-weight ratio isn’t how SG is defined, and viscosity is a separate property entirely.