What is the best definition of specific gravity?

Definition of specific gravity: Sinking vs floating

definition of specific gravity is the ratio of a substance's density to the density of a reference material—typically water for liquids and solids. Because it's a pure ratio without units, SG > 1 means the substance sinks, and SG < 1 means it floats. That's the best definition because it directly links to real-world behavior.

What Is Specific Gravity? The Simple Definition

The definition of specific gravity is the ratio of a substance’s density to the density of a reference substance—typically water for liquids and solids, and air for gases. Because it’s a ratio of two similar densities, specific gravity has no units. A specific gravity above 1 means the material sinks in water; below 1 means it floats. Water itself has a specific gravity of 1 at 4 degrees Celsius, where its density is highest (1 gram per cubic centimeter). ^[1]

That’s the textbook definition. But why does it matter outside a physics classroom? Because specific gravity gives you a quick, portable way to compare materials without needing to remember absolute densities. A shipbuilder knows steel (SG ~7.8) will sink unless shaped to displace enough water—a direct application of Archimedes’ principle. And when a doctor checks your urine, the specific gravity tells them whether your kidneys are concentrating waste properly. So the “best” definition is the one that helps you predict behavior in water.

The Formula and Why Specific Gravity Has No Units

The specific gravity formula and units are straightforward: SG = ρsubstance / ρreference. For liquids and solids, ρ_reference is the density of water, usually taken at 4 degrees Celsius (1.000 g/cm³). For gases, the reference is air at standard temperature and pressure. Because you’re dividing one density by another, the units cancel—leaving a pure number.

Here’s where it gets confusing for many. A density of 2.7 g/cm³ sounds like an absolute value, but a specific gravity of 2.7 simply means the material is 2.7 times denser than water. That distinction tripped me up in college: I kept asking, 'If it has no units, how do you measure it?' The answer: how to calculate specific gravity is to measure density, then divide by water’s density. The hydrometer does this for you—it floats higher in denser liquids, and the scale reads SG directly.

Specific Gravity vs. Density: Clearing Up the Confusion

People often confuse specific gravity vs density, but they’re not the same. Density is an absolute measurement of mass per unit volume; specific gravity is a relative comparison. That difference changes how you use them in the real world.

Density vs. Specific Gravity – A Quick Comparison

Here’s how they differ in practice:

Density

• Changes with both material expansion and reference

• Always has units (g/cm³, kg/m³, lb/ft³)

• When you need an exact mass or volume for engineering calculations

• Mass per unit volume (e.g., 7.8 g/cm³ for steel)

Specific Gravity

• Varies mainly because reference density changes; often reported with temperature

• Dimensionless – no units at all

• Quick comparison, buoyancy prediction, and quality control (batteries, brewing)

• Ratio of substance density to reference density (e.g., steel SG ~7.8)

Using a Hydrometer in a Home Brewery

Carlos, an amateur brewer in Portland, started his first batch of stout with high hopes. He’d followed the recipe exactly, but after three days of fermentation, the airlock was quiet. He worried the yeast had died. A quick gravity reading with his hydrometer showed 1.040—barely changed from the original 1.052.

Panicked, he nearly added more yeast. Instead, he recalled that temperature affects hydrometer readings. His sample was still at 75°F, not the calibrated 60°F. After correcting with an online calculator, the true gravity was 1.046—the yeast was working, just slowly.

He let it ferment another four days. Final gravity landed at 1.012. Using the formula (original – final) × 131, he calculated an ABV of 5.2%. Without the gravity readings, he’d have either bottled prematurely (risking bottle bombs) or dumped a perfectly good beer.