Pour water through sand and it drains; pour it through clay and it puddles — the difference is permeability, the rock's willingness to let fluid pass. Darcy's law captures it: permeability k equals the flow rate times viscosity times length, divided by area times the pressure difference, written k = (q × μ × L) / (A × ΔP). A high k means an easy path, which is why oil companies pay geologists to map it before drilling a single well.

This calculator runs Darcy's law directly. Type the Flow Rate (q, m³/s) from your pump test, the Fluid Viscosity (μ, Pa·s) — about 0.001 for water at room temperature — the Length of Medium (L, m) of your sample, the Cross-sectional Area (A, m²), and the Pressure Difference (ΔP, Pa) across it. The Permeability (k, m²) box returns the answer; sandstone typically lands near 10^-12 m², while tight shale can be a million times lower. A water-well driller in Arizona uses the same math to predict whether a new well will yield gallons per minute or disappointment, and environmental engineers use it to track how fast a spill migrates through soil.

With the pre-filled defaults — Flow Rate (q, m³/s) 0.0001, Fluid Viscosity (μ, Pa·s) 0.001, Length of Medium (L, m) 0.1, Cross-sectional Area (A, m²) 0.01, Pressure Difference (ΔP, Pa) 100,000 — the Permeability Calculator returns Permeability (k, m²) 0.0000000000100000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.