Calculate the specific gravity (SG) of a substance relative to a reference fluid (usually water or air).
Last updated: March 2026 | By ForgeCalc Engineering
Specific gravity (SG) is the ratio of the density of a substance to the density of a reference material. For liquids and solids, the reference is almost always water at its densest temperature (4°C). For gases, the reference is usually dry air at standard temperature and pressure.
Since it is a ratio of two densities, specific gravity is a dimensionless quantity. It is a convenient way to compare the "heaviness" of different materials without worrying about units of measurement.
Where:
• SG is the specific gravity
• ρ_substance is the density of the material (kg/m³)
• ρ_reference is the density of the reference fluid (kg/m³)
Water reaches its maximum density (exactly 1000 kg/m³) at approximately 4°C. Using this as a reference ensures consistency in scientific calculations.
If a substance has an SG greater than 1, it is denser than the reference fluid. For example, steel (SG ≈ 7.8) will sink in water (SG = 1).
Gold has a very high specific gravity of about 19.3. This means it is 19.3 times denser than water, making it easy to distinguish from 'fool's gold' (pyrite, SG ≈ 5.0).
Specific gravity can be measured using a hydrometer (for liquids), a pycnometer, or by using Archimedes' principle to compare the weight of the substance in air vs. in water.
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