Darcy's Law states that the volumetric flow rate Q through a porous medium equals the hydraulic conductivity K multiplied by the hydraulic gradient i and the cross-sectional area A: Q = KiA. It is valid for laminar, slow flow in saturated soils and rocks.

What is the difference between hydraulic conductivity and permeability?

Hydraulic conductivity K (m/s) depends on both the porous medium and the fluid (water). Intrinsic permeability k (m²) depends only on the medium. They are related by k = Kμ/(ρg), where μ is dynamic viscosity, ρ is fluid density, and g is gravity.

What are typical K values for different soils?

Clean gravel: 10⁻² to 1 m/s; coarse sand: 10⁻⁴ to 10⁻² m/s; fine sand: 10⁻⁶ to 10⁻⁴ m/s; silt: 10⁻⁸ to 10⁻⁶ m/s; clay: less than 10⁻⁹ m/s.

When does Darcy's Law not apply?

Darcy's Law breaks down at high flow velocities (Reynolds number > ~10), in unsaturated soils, or in fractured rock where turbulent flow occurs. It is also not applicable when flow is non-Newtonian.

How is hydraulic gradient defined?

Hydraulic gradient i = Δh / L, where Δh is the difference in hydraulic head (total head) between two points and L is the distance between them along the flow path. It is dimensionless.

Permeability Darcy Calculator

Name: Permeability Darcy Calculator
Availability: InStock
Author: Nham Vu

Compute flow rate, hydraulic conductivity, or hydraulic gradient for saturated porous media using Darcy's Law.

Input Parameters

Solve for

Units: SI (m, m/s) Imperial (ft, ft/s)

Hydraulic conductivity K (m/s)

Clay <10⁻⁹ · Silt ~10⁻⁷ · Sand ~10⁻⁵ · Gravel >10⁻³

Hydraulic gradient i (dimensionless)

Enter i directly Head & length

Cross-sectional area A (m²)

Result

Enter values and click Calculate.

Typical Hydraulic Conductivity Values

Material	K (m/s)	Drainage
Clean gravel	10⁻² – 1	Excellent
Coarse sand	10⁻⁴ – 10⁻²	Good
Fine sand	10⁻⁶ – 10⁻⁴	Moderate
Silt	10⁻⁸ – 10⁻⁶	Poor
Sandy clay	10⁻⁹ – 10⁻⁸	Very poor
Clay	<10⁻⁹	Impermeable

Summary