October 11, 2025

What Is The Difference Between Specific Gravity And Density

Q: Can specific gravity have units?

No, specific gravity is a ratio of two densities, so the units cancel out, making it a dimensionless quantity.

Q: Is specific gravity the same as relative density?

Yes, 'specific gravity' and 'relative density' are synonymous terms, both referring to the ratio of a substance's density to that of a reference substance.

Q: Why is water often used as the reference substance for specific gravity?

Water is commonly used as a reference because it is abundant, easily accessible, and its density at 4°C (its maximum density) is almost exactly 1 g/cm³ or 1000 kg/m³, which simplifies calculations.

Q: If a substance has a specific gravity of less than 1, what does that mean?

A specific gravity of less than 1 indicates that the substance is less dense than the reference substance (usually water) and will therefore float on it.

Explore the fundamental differences between specific gravity and density, two crucial concepts in science used to describe the properties of substances. Learn how they are measured and applied.

Have More Questions →

What is the Difference Between Specific Gravity and Density?

Density is an intrinsic property of a substance, defined as its mass per unit volume (e.g., g/cm³ or kg/m³). Specific gravity, on the other hand, is a dimensionless ratio comparing the density of a substance to the density of a reference substance, typically water at a specific temperature (usually 4°C or 20°C). While both relate to how 'heavy' a substance is for its size, density is an absolute measurement, whereas specific gravity is a relative comparison.

Key Principles and Calculation

Density (ρ) is calculated as ρ = m/V, where 'm' is mass and 'V' is volume. It directly tells you how much matter is packed into a given space. Specific gravity (SG) is calculated as SG = ρ_substance / ρ_reference, often with the density of water as the reference (approximately 1 g/cm³ or 1000 kg/m³). This means a substance with an SG of 1.5 is 1.5 times denser than water, and a substance with an SG of 0.8 is 0.8 times denser than water.

Practical Examples of Application

For example, lead has a density of approximately 11.3 g/cm³. Its specific gravity would be 11.3 (since 11.3 g/cm³ / 1 g/cm³ = 11.3). If a liquid has a density of 0.8 g/cm³, its specific gravity is 0.8. Engineers use density to calculate the weight of materials for construction, while specific gravity is often used in hydrometry to measure the concentration of solutions (like in car battery acid or brewing) or to determine if an object will float or sink in water (SG < 1 floats, SG > 1 sinks).

Why the Distinction is Important

The distinction is important because specific gravity offers a convenient way to compare densities without needing to specify units, making it universally applicable across different measurement systems. It's particularly useful when dealing with liquids and solids whose densities are often referenced against water, simplifying comparisons for buoyancy or mixture calculations. Density provides the absolute value, essential for mass-volume calculations, while specific gravity offers a standardized, unitless comparative value.

Frequently Asked Questions

Can specific gravity have units?

Is specific gravity the same as relative density?

Why is water often used as the reference substance for specific gravity?

If a substance has a specific gravity of less than 1, what does that mean?