Geology & Earth ScienceEarth MaterialsGCSE

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Porosity

The percentage of void space in a rock or sediment.

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ϕ = \frac{V _{v o i d}}{V _{t o t a l}} \times 100

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Core idea

Overview

Porosity quantifies the empty space within a material, representing the fraction of the total volume that is not occupied by solid particles. In geology, it's crucial for understanding fluid storage and flow in rocks and sediments, making it a fundamental property of aquifers and hydrocarbon reservoirs, and directly influences a material's ability to hold fluids.

When to use: Use this equation when determining the fractional void space within a bulk material sample, such as rock cores, soil samples, or sediment packs. It's particularly useful for unconsolidated materials or porous solids where the void volume can be directly measured or inferred, and assumes V_total is the total bulk volume including both solids and voids.

Why it matters: Porosity directly controls the storage capacity of geological formations for fluids like groundwater, oil, and natural gas, impacting water resource management and energy exploration. It also influences the rate at which fluids can migrate through the subsurface, which is critical for contaminant transport studies and geothermal energy projects.

Symbols

Variables

\phi = Porosity, V_v = Void Volume, V_t = Total Volume

ϕ

Porosity

V_{v}

Void Volume

c m^{3}

V_{t}

Total Volume

c m^{3}

Walkthrough

Derivation

Formula: Porosity

Definition of rock porosity as a percentage of total volume.

Void volume includes all pores, cracks, and vesicles.

State the definition:

The ratio of space not occupied by solids to the total volume of the rock sample.

ϕ = \frac{V _{v o i d}}{V _{t o t a l}} \times 100

Result

ϕ = \frac{V _{v o i d}}{V _{t o t a l}} \times 100

Source: GCSE Geology — Earth Materials

Free formulas

Rearrangements

Solve for $V_{v}$

Make Vv the subject

V_{v} = \frac{ϕ V _{t}}{100}

To make $V_{v}$ the subject, first clear the denominator $V_{t o t a l}$ , then divide by $100$ .

Difficulty: 2/5

Solve for $V_{t}$

Make Vt the subject

V_{t} = \frac{100 V _{v}}{ϕ}

To make $V_{t}$ (Total Volume) the subject of the porosity formula, first clear the denominator by multiplying by $V_{t o t a l}$ , then divide by $ϕ$ (Porosity).

Difficulty: 2/5

Solve for $ϕ$

Make phi the subject

ϕ = \frac{V _{v}}{V _{t}} \times 100

This rearrangement simplifies the notation of the variables in the formula for porosity, where $ϕ$ is already the subject.

Difficulty: 2/5

The static page shows the finished rearrangements. The app keeps the full worked algebra walkthrough.

Visual intuition

Graph

The graph is a straight line passing through the origin with a slope determined by the total volume. This linear relationship means that doubling the void volume results in a proportional doubling of porosity. For a geology student, this indicates that larger x-values represent rocks with more open space, while smaller x-values represent denser, more compact materials. The most important feature is that the line passes through the origin, meaning that zero void volume results in zero porosity.

Graph type: linear

Why it behaves this way

Intuition

Imagine a sponge: the water it holds fills the interconnected holes (voids), and porosity measures how much of the sponge's total volume is made up of those holes.

ϕ

The fraction of the total volume of a material that is occupied by void space, typically expressed as a percentage.

It tells you how much 'empty room' there is inside a rock or soil for fluids like water or oil.

V_{v o i d}

The volume of the empty spaces, pores, or fractures within a material.

This is the actual amount of space that fluids can potentially fill.

V_{t o t a l}

The total bulk volume of the material sample, encompassing both the solid matrix and the void spaces.

This is the overall size of the sample you are examining, acting as the reference for the void space.

Free study cues

Insight

Canonical usage

The equation calculates a dimensionless ratio of volumes, typically expressed as a percentage by multiplying by 100.

Common confusion

A common mistake is using inconsistent units for $V_{v}$ oid and $V_{t}$ otal (e.g., cubic centimeters for void volume and cubic meters for total volume), which will lead to an incorrect and dimensionally inconsistent result.

Dimension note

Porosity is a dimensionless quantity, representing a ratio of two volumes. Although the formula includes multiplication by 100 to express it as a percentage, the underlying physical quantity is a pure number.

Unit systems

$ϕ$ % · Porosity is a dimensionless quantity, often reported as a percentage (e.g., 20%), indicating the fraction of void space relative to the total volume.

$V_{v o i d}$ m3 | cm3 | L · The volume of the void space within the material. Must be in the same units as V_total for the ratio to be dimensionless.

$V_{t o t a l}$ m3 | cm3 | L · The total bulk volume of the material, including both solid particles and void space. Must be in the same units as V_void for the ratio to be dimensionless.

Ballpark figures

Quantity:

One free problem

Practice Problem

A rock sample has a total volume of 250 cm³ and its void spaces account for 75 cm³. Calculate the porosity of this sample.

Total Volume250 cm³

Void Volume75 cm³

Solve for: $p hi$

Hint: Divide the void volume by the total volume before multiplying by 100 to express as a percentage.

The full worked solution stays in the interactive walkthrough.

Where it shows up

Real-World Context

A sandstone block has a total volume of 100cm³ and 20cm³ of air space. Porosity = 20%.

Study smarter

Tips

Always ensure consistent units for $V_{v}$ oid and $V_{t}$ otal.
Remember porosity is often expressed as a percentage.
Distinguish between total and effective porosity.
Consider grain packing and sorting when estimating porosity.

Avoid these traps

Common Mistakes

Forgetting to multiply by 100.
Confusing porosity with permeability (flow rate).

Common questions

Frequently Asked Questions

Definition of rock porosity as a percentage of total volume.

Use this equation when determining the fractional void space within a bulk material sample, such as rock cores, soil samples, or sediment packs. It's particularly useful for unconsolidated materials or porous solids where the void volume can be directly measured or inferred, and assumes V_total is the total bulk volume including both solids and voids.

Porosity directly controls the storage capacity of geological formations for fluids like groundwater, oil, and natural gas, impacting water resource management and energy exploration. It also influences the rate at which fluids can migrate through the subsurface, which is critical for contaminant transport studies and geothermal energy projects.

Forgetting to multiply by 100. Confusing porosity with permeability (flow rate).

A sandstone block has a total volume of 100cm³ and 20cm³ of air space. Porosity = 20%.

Always ensure consistent units for V_void and V_total. Remember porosity is often expressed as a percentage. Distinguish between total and effective porosity. Consider grain packing and sorting when estimating porosity.

References

Sources

Wikipedia: Porosity (geology)
Earth Science (15th Edition) by Tarbuck, Lutgens, and Tasa
Wikipedia: Porosity
IUPAC Gold Book: Porosity
Bird, R. Byron, Stewart, Warren E., Lightfoot, Edwin N. Transport Phenomena.
Incropera, Frank P., DeWitt, David P., Bergman, Theodore L., Lavine, Adrienne S. Fundamentals of Heat and Mass Transfer.
IUPAC Gold Book, 'porosity'
Wikipedia, 'Porosity'

Porosity

Overview

Variables

Derivation

State the definition:

Rearrangements

Graph

Intuition

Insight

Practice Problem

Real-World Context

Tips

Common Mistakes

Related Formulas

Darcy's Law (Specific Discharge)

Frequently Asked Questions

Sources