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

Edited by Aimie Carlson || By Janet White || Published on March 10, 2024
Bulk density measures mass per unit volume of a material in its entirety, including voids. Specific gravity compares the density of a material to that of water, without considering voids.

## Key Differences

Bulk density and specific gravity are two important concepts used to describe the characteristics of materials, but they serve different purposes and are measured differently. Bulk density refers to the weight of a material per unit volume, including the spaces (voids) between particles. Specific gravity, on the other hand, is a dimensionless quantity that compares the density of a material to the density of water at a specified temperature (usually 4°C, where water has its maximum density).
The primary difference between these two measurements lies in their consideration of the material's voids. Bulk density accounts for the entire volume of a substance, including the internal and external voids, which is why it is lower for materials with high porosity. Specific gravity focuses on the density of the material itself, excluding any voids, which makes it a useful measure for comparing the inherent properties of different substances.
In practical terms, bulk density helps in determining the storage, handling, and transport requirements of a material, as it directly relates to the volume a given mass of the material will occupy. Specific gravity, by ignoring void spaces, is critical in applications where the material's relative density affects its performance, such as in fluid dynamics and material science.
Both bulk density and specific gravity are vital for engineers, scientists, and manufacturers, as they provide essential data for material selection, product formulation, and quality control. Understanding the distinction between these measurements helps professionals in various fields make informed decisions based on the physical properties of materials.

## Comparison Chart

### Definition

Mass of a material per unit volume, including voids.
Ratio of the density of a material to that of water.

### Units

Kilograms per cubic meter (kg/m³) or grams per cubic centimeter (g/cm³).
Dimensionless (ratio).

### Considers Voids

Yes, accounts for void spaces within the material.
No, ignores void spaces, focusing on material density.

### Application

Useful in determining packing, storage, and transportation.
Important for material identification and behavior in fluids.

### Measurement

Measured directly through mass and volume.
Calculated by comparing the material's density to water's.

## Bulk Density and Specific Gravity Definitions

#### Bulk Density

Indicates the compactness of granular materials.
High bulk density in fertilizers suggests fewer voids and a more solid product.

#### Specific Gravity

A ratio comparing the density of a material to that of water.
Gold's specific gravity of 19.3 means it's 19.3 times denser than water.

#### Bulk Density

Measured in kilograms per cubic meter or grams per cubic centimeter.
We calculated the bulk density to be 1.2 g/cm³, indicating a fairly compact material.

#### Specific Gravity

Dimensionless, indicating it has no units.
The specific gravity of ethanol is 0.789, which is less than water.

#### Bulk Density

The weight of a material including its void spaces per unit volume.
The bulk density of the soil influences its water retention capacity.

#### Specific Gravity

Determines whether an object will float or sink in water.
Objects with a specific gravity less than 1 will float in water.

#### Bulk Density

Essential for construction materials to assess volume needs.
The bulk density of gravel is crucial for calculating how much is needed for the foundation.

#### Specific Gravity

Important for identifying substances in chemistry.
We identified the liquid as glycerin by its specific gravity.

#### Bulk Density

Affects the storage and shipping of agricultural products.
Grain's bulk density determines the size of silos required for storage.

#### Specific Gravity

Used in the design of ships and submarines for buoyancy.
The engineer considered the specific gravity of materials to ensure proper buoyancy.

## FAQs

#### What does bulk density tell you?

It indicates how compact a material is, including its void spaces.

#### How do porosity and compaction influence bulk density?

Higher porosity increases voids, lowering bulk density; compaction reduces voids, increasing bulk density.

#### Can specific gravity be greater than 1?

Yes, if a material is denser than water, its specific gravity will be greater than 1.

#### What affects bulk density?

The arrangement and size of particles, as well as the presence of voids.

#### What unit is used for specific gravity?

Specific gravity is a ratio and therefore dimensionless, without units.

#### Why is specific gravity important?

It helps compare the density of materials to water, affecting their behavior in fluids.

#### How do you measure bulk density?

By dividing the mass of a material by its total volume, including voids.

#### Is bulk density the same for all forms of a material?

No, it can vary depending on how the material is processed or compacted.

#### Why might two materials with the same bulk density have different specific gravities?

Because specific gravity does not consider void spaces, while bulk density does.

#### Are specific gravity and density the same?

No, specific gravity is a ratio of densities, while density is mass per unit volume.

#### Does temperature affect specific gravity?

Yes, since the density of materials and water can change with temperature.

#### What is the specific gravity of water?

By definition, the specific gravity of water is 1 at 4°C.

#### Can specific gravity help in determining material purity?

Yes, deviations from a known specific gravity can indicate impurities.

#### How do you find specific gravity?

By dividing the density of the material by the density of water.

#### Is high specific gravity always desirable?

It depends on the application; for buoyancy, a lower specific gravity is preferable.

#### Does the shape of a material affect its bulk density?

Yes, shape can influence how materials pack together, affecting void space.

#### What role does bulk density play in soil health?

It affects water and air movement in soil, influencing plant growth.

#### How does moisture content affect bulk density?

Increasing moisture content can increase bulk density by filling void spaces.

#### Can the specific gravity of a liquid be measured the same way as a solid?

Yes, using methods like a hydrometer or pycnometer.

#### What is a typical specific gravity for rocks and minerals?

Most rocks have specific gravities between 2.5 and 3.0.