Specific gravity — also called relative density — compares how heavy a material is against a reference, almost always water. It is simply the substance's density divided by water's density, SG = ρ_substance / ρ_water, and because the units cancel the result is a pure number. That makes it instantly readable: anything above 1 is denser than water and sinks, anything below 1 floats. And since water is close to 1 g/cm³, a substance's specific gravity matches its density in g/cm³ — a handy shortcut hydrometers exploit.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the definition of relative density referenced to water at 4 °C.
The specific gravity equations
The reference density goes in the denominator. With water as the reference, SG and density in g/cm³ are numerically identical, while in kg/m³ the density is 1000 × SG. To recover an unknown density, multiply the specific gravity by the reference density; to find the reference, divide the substance density by SG. Always keep both densities in the same unit so the ratio stays clean and dimensionless.
Worked example — a dense brine
Scenario: A concentrated brine has a density of 1250 kg/m³. Water is 1000 kg/m³. What is its specific gravity, and will it sink in fresh water?
The brine's specific gravity is 1.25 — it is 25% denser than water, so it sinks. Working in g/cm³ instead of kg/m³ gives exactly the same 1.25, confirming the ratio is unit-independent. By contrast ethanol at 790 kg/m³ gives SG = 0.79, below 1, so it would float on water — the same comparison a hydrometer makes when it bobs higher or lower in a liquid.
Frequently Asked Questions
SG = substance density ÷ reference density (water 1000 kg/m³). 1250 / 1000 = 1.25. It's unitless.
Density has units; SG is the ratio to water, so dimensionless. SG equals density in g/cm³.
SG > 1 sinks, SG < 1 floats, SG = 1 is neutral. Ice (0.92) floats; mercury (13.6) sinks.
It's stable and ≈ 1 g/cm³ at 4 °C, so SG = density in g/cm³. Gases use air instead.
Yes — oils, ethanol (0.79), wood and ice are all below 1 and float on water.