Permeability Calculator
Porosity Tells You How Much Space Exists; Permeability Tells You Whether Fluid Can Move Through It
A rock can be highly porous and still transmit almost no fluid at all, if that pore space isn't connected. Shale is a classic example: it can hold a meaningful volume of water or hydrocarbons in its pores, yet those pores are so poorly connected that almost nothing flows through it. Permeability, derived from Darcy's Law, is the property that actually governs flow — and it's what separates a productive aquifer or reservoir from a sealed one.
The Formula
Solved for permeability: k = (Q·μ·L) ÷ (A·ΔP)
Q is flow rate, A is cross-sectional area, ΔP is the pressure difference driving flow, μ is fluid viscosity, and L is the flow path length. Rearranging Darcy's Law isolates k, the permeability coefficient, directly from measurable flow-test quantities.
Why Permeability Is the Property That Matters
- Aquifer productivity — a well's sustainable pumping rate depends on the permeability of the surrounding formation, not just how porous it is.
- Petroleum reservoir engineering — oil and gas only flow toward a wellbore at commercially useful rates if the reservoir rock's permeability is high enough; this is a central input to reservoir simulation.
- Contaminant containment — landfill liners and containment barriers are specifically engineered for very low permeability, to slow leachate movement into surrounding soil and groundwater.
- Seal and cap-rock evaluation — a low-permeability layer above a reservoir or aquifer is what keeps fluids trapped in place over geologic time.
Relative Permeability by Material
| Material | Relative Permeability |
|---|---|
| Gravel | Very high |
| Clean sand | High |
| Sandstone (well-sorted) | Moderate |
| Silt | Low |
| Limestone (unfractured) | Low |
| Clay | Very low |
| Unfractured granite | Extremely low |
This qualitative ordering is a widely published hydrogeology reference ranking; the exact numeric permeability value this calculator returns depends entirely on the flow-test units you enter.
How to Use This Calculator
- Enter Flow Rate Q (e.g. cm³/s).
- Enter Cross-Sectional Area A (cm²).
- Enter Pressure Difference ΔP (e.g. atm).
- Enter Fluid Viscosity μ (e.g. cP).
- Enter Length L (cm), the distance over which flow occurs.
- Select Calculate to get the permeability coefficient k, with Darcy's Law applied to your figures.
Related Calculations
Check how much pore space is available in the first place with the Porosity Calculator, or evaluate load-bearing behavior with the Soil Bearing Capacity Calculator.