Specific Gravity

Specific gravity is a dimensionless ratio that compares the density of a substance to the density of a reference substance — almost always water, measured at 1000 kg/m³ (1 g/cm³) at 4°C. Rather than expressing how heavy something is in absolute units like kilograms per cubic meter, specific gravity simply says "how many times denser than water is this," stripping away the unit system entirely.

The Specific Gravity Calculator computes this ratio directly, and pairs naturally with the Density Calculator, which measures the underlying absolute density in kg/m³ or g/cm³ that specific gravity is built from. Because it's a pure ratio, specific gravity is often more convenient for practical, everyday comparisons — brewers use it to track fermentation, mechanics use it to check battery acid, and doctors use it to assess urine concentration, all without needing to think in absolute density units.

Specific gravity and Density describe the same underlying physical property from two different angles: density is the absolute measurement, and specific gravity is that measurement normalized against water. A liquid with a specific gravity of 1.05 has a density of 1050 kg/m³, exactly 5% more than water.

SG = ρ_substance / ρ_water

Where SG is specific gravity (dimensionless — no units), ρ_substance is the density of the substance being measured, and ρ_water is the density of water, conventionally taken as 1000 kg/m³ (or 1 g/cm³) at 4°C. Because the same units appear in both the numerator and denominator, they cancel out completely.

A sample of seawater has a measured density of 1025 kg/m³. Its specific gravity relative to fresh water is:

SG = 1025 kg/m³ ÷ 1000 kg/m³ = 1.025

This means seawater is 2.5% denser than fresh water — the reason objects float more easily in the ocean than in a freshwater lake, and a figure regularly used by marine engineers to calculate buoyancy for ship design.

• Always dimensionless: because specific gravity is a ratio of two densities in the same units, the units always cancel, leaving a pure number that's the same whether you started in kg/m³ or g/cm³.
• Directly predicts floating or sinking: any specific gravity value under 1.0 means the substance is less dense than water and will float, while a value over 1.0 means it will sink.
• Built from Density: specific gravity is simply the ratio of a substance's density to water's density, so any density measurement can be converted into a specific gravity figure and vice versa.
• Widely used in quality testing: brewers track specific gravity to measure sugar content before and after fermentation, mechanics check battery acid specific gravity to gauge charge state, and clinicians use urine specific gravity as a hydration and kidney-function indicator.
• Reference temperature matters: because water's density shifts slightly with temperature, precise specific gravity figures specify the temperature at which both the sample and the reference water were measured, typically 4°C or 20°C.