Hazen Formula for Permeability from Grain Size
The Hazen formula gives a first estimate of hydraulic conductivity from nothing more than the finest ten percent of a grading curve.
The equation
K = C (d10)²
- K is hydraulic conductivity in centimeters per second
- d10 is the grain size in millimeters that ten percent of the sample passes
- C is a coefficient near 1.0 for clean uniform sand
Worked example
A clean sand with d10 of 0.2 millimeters gives K = 1.0 × 0.04 = 0.04 centimeters per second, which is roughly 35 meters per day, squarely in the range of a productive sand aquifer.
When to use it
A screening tool for clean sands with d10 between about 0.1 and 3 millimeters. It overestimates badly in silty or well graded soils, so confirm with a permeameter or field test before designing anything.
Related Calculators and Reference
- Groundwater Flow Simulator: watch this equation animate a cone of depression.
- Demo Valley Worked Project: a full site project using these methods.
- USGS Water Science School: authoritative background on groundwater.