Permeability Coefficient Calculator
Estimate the coefficient of permeability (hydraulic conductivity) of a clean sand from its effective grain size D10 using Hazen's empirical formula. A quick first-pass tool for geotechnical and drainage assessments before a lab permeability test.
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the effective grain size D10 (the 10%-passing sieve size) in millimetres.
- Optionally change the Hazen coefficient C (default 100; 40–150 for clean sand).
- Read the permeability k in cm/s and m/s plus an indicative drainage note.
How it works
Hazen's formula relates permeability to the square of the effective grain size: k (cm/s) = C · D10², with D10 in mm and C the Hazen coefficient (default 100). The result is divided by 100 to give k in m/s.
The formula is a grain-size correlation, valid for fairly uniform clean sands with D10 roughly 0.1–3 mm and a coefficient of uniformity Cu = D60/D10 below about 5. It is not reliable for silts, clays or well-graded soils, and a laboratory falling-head or constant-head test measures k directly when accuracy matters.
Worked example
Uniform medium sand. A clean sand has D10 = 0.2 mm. With C = 100, k = 100 × 0.2² = 4 cm/s = 0.04 m/s — a highly permeable, freely draining sand.
Common mistakes
- Entering D10 in cm or metres — Hazen's C = 100 assumes D10 in millimetres.
- Applying Hazen to silts, clays or well-graded soils (Cu ≥ 5), where it is not valid.
- Forgetting that this is an estimate, not a substitute for a measured falling-head/constant-head permeability test.
Frequently asked questions
What is Hazen's formula for permeability?
k (cm/s) = C · D10², where D10 is the effective grain size in mm and C is the Hazen coefficient (default 100, typically 40–150 for clean sand).
What is D10?
D10 is the effective grain size — the sieve size through which 10% of the soil by mass passes, read from the particle-size distribution curve.
When is Hazen's formula valid?
For fairly uniform clean sands with D10 about 0.1–3 mm and a coefficient of uniformity Cu below roughly 5. It should not be used for silts, clays or well-graded soils.
How do I convert k from cm/s to m/s?
Divide by 100. For example 4 cm/s = 0.04 m/s.
Is Hazen's formula as good as a lab test?
No. It is an empirical correlation from grain size. A falling-head or constant-head permeability test measures k directly and should be used for design.
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