Specific Gravity Calculator
Enter a substance density and a reference density to get the specific gravity — the dimensionless ratio that tells you whether a material sinks or floats in water.
Density ratio at a glance
Enter the substance density and a reference density and the calculator returns the specific gravity (relative density) as a dimensionless number.
Same units both sides
Use the same unit for both densities — kg/m³ is fine, so is g/cm³ — because the result is a ratio, the units cancel and the number is the same.
What is specific gravity?
A density compared to water
The specific gravity calculator turns two densities into a single, unit-free number: the ratio of a substance's density to a reference density. Specific gravity, also called relative density, tells you how heavy a material is compared with a reference — almost always water at 4 °C, which has a density of 1000 kg/m³. Because it divides one density by another, the answer is dimensionless: it carries no units at all, and it stays the same whether you measure in kilograms per cubic metre or grams per cubic centimetre. It is the number behind hydrometer readings, gemstone identification, battery-acid checks, and brewing measurements.
Enter a substance density and a reference density to get the specific gravity (relative density) as a single dimensionless number instantly.
Specific gravity is the substance's density divided by the reference density.
SG = ρ_substance / ρ_referencePick the reference to match the substance: water at 4 °C (1000 kg/m³) for liquids and solids, air for gases. Because both densities sit on the same scale, their units cancel — the result is a pure ratio with no unit attached. A reading of 0.789 means the substance is 78.9 % as dense as water.
Suppose you measure ethanol at a density of 789 kg/m³ and compare it with water at 1000 kg/m³.
Note both densities
Substance density 789 kg/m³, reference density 1000 kg/m³ (water at 4 °C).
Divide substance by reference
789 / 1000 = 0.789 — the densities are on the same scale, so the units cancel.
Read the result
The specific gravity is 0.789 — below 1, so ethanol is lighter than water and floats on it.
The single number does all the work of comparison. A specific gravity greater than 1 means the substance is denser than the reference and will sink in it — mercury (13.5), iron (7.9), and aluminium (2.7) all sink in water. A specific gravity less than 1 means it is lighter and will float: ethanol comes in at 0.789, ice at about 0.917, and most oils sit between 0.8 and 0.95, which is why they float on water. A value of exactly 1 means the substance has the same density as the reference and is neutrally buoyant. Because the number is dimensionless, it is a clean way to compare materials across measurement systems and to spot anomalies — a battery hydrometer reading, for instance, drops below its expected value as the acid weakens, and a gemstone's specific gravity helps confirm what it actually is.
The formula is exact, but a couple of practical points are worth keeping in mind.
Match the reference and watch the temperature
Specific gravity only means something when you state the reference. Water at 4 °C (1000 kg/m³) is the convention for liquids and solids, but densities shift with temperature, so a precise reading needs both densities measured at the same temperature. Use the same unit on both sides — the ratio cancels units, but mixing kg/m³ with g/cm³ gives a wrong number. For gases, compare against air rather than water.