SG = ρ_substance / ρ_water
Specific gravity (SG) is the ratio of a substance's density to a reference (usually water at 4°C, ρ = 0.9998 g/mL). Since it's a ratio, SG is dimensionless and numerically close to density in g/mL for most substances. SG > 1: sinks in water; SG < 1: floats. Examples: concentrated H₂SO₄ (SG = 1.84), ethanol (SG = 0.789), mercury (SG = 13.6), gasoline (SG ≈ 0.75). SG is measured with hydrometers, pycnometers, or digital density meters. Applications: battery acid testing (SG indicates charge state), urine analysis (SG 1.005-1.030 indicates kidney function), brewing (original/final gravity determines alcohol content), petroleum grading (API gravity = 141.5/SG − 131.5), and mineral identification.