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Density vs. Specific Gravity: What's the Difference?

Edited by Janet White || By Harlon Moss || Updated on October 16, 2023
Density measures mass per unit volume, typically in units like kg/m^3. Specific gravity compares an object's density to the density of water, and is dimensionless.

Key Differences

Density is a fundamental property of matter, defined as its mass per unit volume. It tells us how much mass is contained in a given volume of a substance. Specific gravity, on the other hand, is a relative measure, comparing the density of a substance to that of a reference substance, usually water at a specified temperature.
For example, if a metal has a density of 8000 kg/m^3, it means there are 8000 kilograms of that metal in a cubic meter. Specific gravity would tell us how that metal's density compares to water's density (1000 kg/m^3 at 4°C). If the metal's specific gravity is 8, it's eight times denser than water.
Density is used across various fields like physics, engineering, and geology to determine how much of a substance is packed into a given space. Specific gravity is widely utilized in industries like brewing and gemology to determine relative densities.
While density provides absolute values with distinct units, specific gravity provides a ratio, and thus, it is a dimensionless number. This makes specific gravity particularly useful in practical applications, such as determining if an object will float in water.
An interesting point of note is that specific gravity can change based on the reference substance. While water is the most common reference, other substances can be used depending on the context. Density, however, remains a constant, inherent property of a material.

Comparison Chart


Mass per unit volume of a substance.
Ratio of the density of a substance to a reference substance (usually water).


Typically in kg/m^3, g/cm^3, etc.
Dimensionless (no units) as it's a ratio.


Absolute measurement of a material's compactness.
Relative to a reference substance.

Use Cases

Physics, engineering, geology, etc.
Brewing, gemology, fluid mechanics, etc.

Dependency on Reference

Not dependent on any reference material.
Dependent on reference material, typically water.

Density and Specific Gravity Definitions


Density represents how compact the matter in an object is.
A sponge has a low density because it has many air pockets.

Specific Gravity

Specific gravity is often used for identifying substances.
Gemologists use specific gravity to differentiate between similar-looking gemstones.


Density is the mass of an object divided by its volume.
Lead has a high density, making it very heavy for its size.

Specific Gravity

Specific gravity helps determine buoyancy in fluids.
Objects with a specific gravity greater than 1 will sink in water.


Density can be influenced by temperature and pressure.
Cold air has a higher density than warm air, causing it to sink.

Specific Gravity

Specific gravity provides a dimensionless comparison.
If a gemstone has a specific gravity of 3, it's three times denser than water.


Density is an intrinsic property of materials.
The density of gold is consistent, which aids in authenticating genuine gold items.

Specific Gravity

Specific gravity is the ratio of a substance's density to the density of a reference substance.
The specific gravity of gold is around 19.32 when compared to water.


Density has units related to mass and volume.
Water has a density of 1 g/cm^3, making it a benchmark for many comparisons.

Specific Gravity

Specific gravity changes if the reference substance changes.
Specific gravity values will differ if referenced to substances other than water.


The quality or condition of being dense.


The quantity of something per unit measure, especially per unit length, area, or volume.


Is specific gravity always related to water?

Typically yes, but other reference substances can be used based on context.

What is density?

Density is the amount of mass in a specified volume of a substance.

How is specific gravity different from density?

Specific gravity is a ratio comparing a substance's density to a reference substance's density, usually water.

How is specific gravity used in brewing?

It helps measure the sugar concentration in wort, indicating fermentation progress.

What tools measure specific gravity?

Hydrometers and pycnometers are commonly used tools.

Does temperature affect density?

Yes, temperature can cause expansion or contraction, affecting density.

Is specific gravity a constant for a substance?

Generally, yes, for a given temperature and pressure, unless the reference substance changes.

Why is water often used as the reference in specific gravity?

It's a common, consistent substance with a conveniently rounded density value of 1 g/cm^3 or 1000 kg/m^3.

Can specific gravity be more than 1?

Yes, values greater than 1 indicate substances denser than the reference.

Do all substances with the same density have the same specific gravity?

Yes, if the reference substance is the same.

What unit is density typically measured in?

Common units include kg/m^3 and g/cm^3.

Why is specific gravity dimensionless?

It's a ratio of two densities, so units cancel out.

Can gases have density values?

Yes, gases have density, but they're generally much lower than liquids and solids.

Are density and specific gravity related to buoyancy?

Yes, both play roles in determining whether objects sink or float in fluids.

Can two materials with different densities have the same specific gravity?

No, specific gravity directly relates to density when referenced to the same substance.

Can two objects have the same specific gravity but different densities?

No, if they have the same specific gravity relative to the same reference, their densities are identical.

Why is density important in physics?

It helps understand properties like buoyancy, pressure, and fluid dynamics.

Does pressure influence density?

Yes, increased pressure generally increases density by decreasing volume.

How does density relate to weight?

Density multiplied by volume gives mass, and weight is mass under gravity.

What's the significance of a specific gravity value less than 1?

It indicates the substance is less dense than the reference, often meaning it'll float.
About Author
Written by
Harlon Moss
Harlon is a seasoned quality moderator and accomplished content writer for Difference Wiki. An alumnus of the prestigious University of California, he earned his degree in Computer Science. Leveraging his academic background, Harlon brings a meticulous and informed perspective to his work, ensuring content accuracy and excellence.
Edited by
Janet White
Janet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.

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