Porosity and Permeability Calculator

Porosity and Permeability in Porous Media

Porosity measures how much void space exists in a material relative to its total bulk volume. Permeability measures how easily a fluid can flow through that material under a pressure gradient. Both properties are central to hydrogeology, petroleum engineering, and soil science.

Porosity Formula

$$ \phi = \frac{V_{\text{void}}}{V_{\text{bulk}}} $$

Porosity is dimensionless and often expressed as a percentage. A value of 0.30 means 30% of the bulk volume is void space.

Darcy's Law for Permeability

$$ Q = \frac{kA}{\mu L} \Delta p $$

Rearranged to solve for permeability:

$$ k = \frac{Q \mu L}{A \, \Delta p} $$

$Q$ is discharge rate, $k$ is permeability, $A$ is cross-sectional area, $\mu$ is dynamic viscosity, $L$ is flow path length, and $\Delta p$ is pressure difference across the sample.

Practical Notes

High porosity does not always mean high permeability. If voids are isolated, fluid cannot flow even when porosity is large. Sandy soils tend to have higher permeability than clay-rich soils.

Frequently Asked Questions

What is the difference between porosity and permeability?

Porosity is the fraction of void volume. Permeability describes how well connected voids allow fluid flow.

What units does permeability use?

SI permeability is m². Geology often uses the darcy (1 darcy ≈ 9.87 × 10⁻¹³ m²) or millidarcy.

What porosity do soils typically have?

Sandy soils often range from 0.36 to 0.43. Clay-rich soils can exceed 0.50 porosity but may still flow poorly.

When should I use Darcy's law?

Darcy's law applies to slow, laminar flow through porous media such as groundwater movement through aquifers.

Can porosity exceed 1?

No. Porosity is the ratio of void volume to bulk volume, so valid values lie between 0 and 1.

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