Calculate hydraulic conductivity K, Darcy velocity, seepage velocity, and total flow for soils and aquifers. Presets for gravel through clay.
Hydraulic conductivity K describes how easily water moves through a porous medium under a hydraulic gradient. It is the proportionality constant in Darcy's law, q = K × i, and varies over more than ten orders of magnitude, from clean gravels (K ≈ 10⁻² m/s) to intact clays (K ≈ 10⁻⁹ m/s).
This calculator computes Darcy velocity (specific discharge), seepage velocity (accounting for porosity), and total volumetric flow through a cross-section. You can either specify K from soil-type presets or back-calculate it from measured flow, gradient, and area, which is the usual outcome of a field or lab permeability test.
Practical applications include groundwater travel-time estimates, dewatering design, well and drain sizing, dam seepage checks, and slope-stability screening. The comparison table shows how the same gradient produces very different flow rates in different soils, which makes the scale of subsurface variability easier to see. It is especially helpful when you need to translate a handbook K value into an actual seepage or discharge estimate before a more detailed site model is available.
Use this calculator when you need to move from a conductivity estimate to velocity and flow without rebuilding Darcy's law for each scenario.
It is useful for groundwater screening, teaching examples, site comparisons, and quick checks of whether a measured K value is plausible for the stated soil or aquifer material.
Darcy's Law: q = K × i Total flow: Q = q × A = K × i × A Seepage velocity: v = q / n Where: • K = hydraulic conductivity (m/s) • i = hydraulic gradient (dimensionless) • A = cross-sectional area (m²) • n = effective porosity • q = specific discharge / Darcy velocity (m/s)
Result: Q = 1×10⁻⁴ m³/s (8.64 m³/day)
q = 5×10⁻⁴ × 0.02 = 1×10⁻⁵ m/s. Q = 1×10⁻⁵ × 10 = 1×10⁻⁴ m³/s. Seepage velocity = 1×10⁻⁵ / 0.3 = 3.3×10⁻⁵ m/s ≈ 2.9 m/day.
Hydraulic conductivity is best treated as a site-specific range, not as a single universal value for a named soil. Grain size, fabric, compaction, layering, and water temperature can all change K materially, so field or lab measurements should override handbook values whenever they are available. The calculator is most useful when it turns that conductivity estimate into a velocity or flow number you can compare with field intuition.
The most common mistakes are mixing conductivity with intrinsic permeability, using total porosity instead of effective porosity for seepage velocity, and assuming the flow path is homogeneous. Layered or anisotropic formations can behave very differently from the simple one-dimensional picture used in a first-pass Darcy calculation.
Darcy velocity (specific discharge) q = K·i is the volumetric flux per unit total area — it treats soil as a continuum. The actual water speed in the pores (seepage velocity) is v = q/n, always larger because water only flows through the pore space (fraction n).
Common methods include slug tests, pumping tests with observation wells, and borehole permeameter tests. Lab methods use constant-head or falling-head permeameters on undisturbed or reconstituted samples.
K = k·ρg/μ. Intrinsic permeability k (m²) is a property of the medium alone. K (m/s) also depends on fluid properties (density, viscosity). For cold water, K ≈ k / 1.0×10⁻⁷.
Use effective porosity (connected pore space, typically 0.15–0.35 for sands). Total porosity is higher but includes dead-end pores that do not conduct flow.
Transmissivity T = K × b, where b is the aquifer thickness. It tells you the flow capacity of the entire aquifer layer and is the key parameter from pumping tests.
Yes. Most real soils have higher horizontal K (Kh) than vertical K (Kv), often by a factor of 2–10. Layered soils can have Kh/Kv ratios of 100+.