Definition
The ratio of the weight (or density) of a substance to the weight (or density) of an equal volume of pure water at a standard temperature. Water is assigned a value of 1.0; substances heavier than water have a specific gravity greater than 1.0, and substances lighter than water have a value less than 1.0.
Plain English
A number that tells you how heavy a liquid is compared to water. Water is 1.0. If a fluid reads higher, it's heavier than water. If it reads lower, it's lighter.
Context Anchor
Seen in aircraft maintenance when checking fluids such as battery liquid, fuel, oil, or other materials against expected values.
Derivation
From Latin specificus ("of a particular kind") and gravitas ("weight"). Together: the particular weight of a substance — but always compared to a standard, which is water.
Why Pilots Care
On aircraft batteries, the specific gravity of the electrolyte tells you the battery's state of charge. A fully charged lead-acid cell typically reads around 1.275 to 1.300; a discharged cell reads much lower. It's a direct, physical check that no voltmeter can fully replace.
Analogy
Think of two identical cups: one filled with water and one filled with another liquid. Specific gravity tells whether the other cup would weigh more, less, or the same as the cup of water.
Intuition Check
Specific gravity does not mean how strongly gravity pulls on something. Here, it means a comparison of how heavy a material is for its size, usually compared with water.
Example Sentence 1
The technician used a hydrometer to check the specific gravity of each battery cell before returning the aircraft to service.
Example Sentence 2
Using the specific gravity of the oil, the technician calculated the exact weight added during servicing.