Specific gravity is one of the simplest ways to describe how dense a substance is without getting tangled in units. By comparing a substance's density to a fixed reference — almost always water — specific gravity turns an awkward number like 1,200 kg/m³ into an intuitive one like 1.2: this substance is 20% denser than water.
Why Divide by a Reference Density?
Density on its own depends entirely on which unit system you're using — kg/m³, g/cm³, lb/ft³ — and the raw numbers can be hard to compare at a glance. Dividing by a fixed reference density cancels the units and produces a single dimensionless number that means the same thing no matter which unit system you started from. A specific gravity of 1.2 always means "20% denser than the reference," whether the underlying density was measured in metric or imperial units.
Choosing the Right Reference: Water vs. Air
Water at 4°C is the near-universal reference for liquids and solids because it's a stable, reproducible standard with a density defined as exactly 1,000 kg/m³ (1 g/cm³). Gases and vapors instead use air at sea level and 20°C (1.225 kg/m³) as their reference, since comparing a gas to water would produce impractically tiny numbers. This calculator lets you switch between the two references so the ratio you get is meaningful for the kind of substance you're measuring.
A Quick Float-or-Sink Test
When the reference is water, specific gravity doubles as a fast float-or-sink check: anything with SG below 1 is less dense than water and floats, and anything above 1 is denser and sinks. This is why ice (SG ≈ 0.92) floats on liquid water while a steel ball bearing (SG ≈ 7.8) sinks immediately. It's an approximation — real buoyancy also depends on shape and surface tension for small or oddly-shaped objects — but for a solid, roughly uniform substance, the SG-vs-1 comparison is a reliable rule of thumb.