Coefficient of Permeability Calculator
Estimate permeability from flow, head, area, and duration. Visualize seepage behavior through plotted comparison points. Plan drainage and soil assessments with stronger engineering confidence.
Input Form
Use consistent units. The calculator converts values to SI internally.
Plotly Graph
The chart updates after each calculation.
Example Data Table
| Trial | Method | Main Inputs | k (m/s) | k (cm/s) |
|---|---|---|---|---|
| A | Constant head | Q=600 mL, L=15 cm, A=78.5 cm2, h=30 cm, t=3 min | 2.1231e-4 | 2.1231e-2 |
| B | Constant head | Q=750 mL, L=15 cm, A=78.5 cm2, h=35 cm, t=3 min | 2.2748e-4 | 2.2748e-2 |
| C | Falling head | a=3.14 cm2, L=15 cm, A=78.5 cm2, h₁=80 cm, h₂=40 cm, t=5 min | 1.3863e-5 | 1.3863e-3 |
| D | Falling head | a=3.14 cm2, L=20 cm, A=100 cm2, h₁=90 cm, h₂=45 cm, t=6 min | 1.2092e-5 | 1.2092e-3 |
Formula Used
Constant head test
Q = k × i × A × t
i = h / L
k = (Q × L) / (A × h × t)
Falling head test
k = (a × L / (A × t)) × ln(h₁ / h₂)
Here, a is standpipe area, A is specimen area, L is specimen length, and h₁ and h₂ are initial and final heads.
The calculator also reports Darcy velocity using v = q / A. When porosity is entered, it estimates seepage velocity from vs = v / n.
Keep all measurements realistic and field-relevant. Laboratory temperature, saturation level, and sample disturbance can influence actual permeability values.
How to Use This Calculator
- Choose the correct test method for your soil sample.
- Enter specimen length, specimen area, elapsed time, and porosity.
- For constant head, provide discharge volume and head loss.
- For falling head, provide standpipe area, initial head, and final head.
- Use the unit dropdowns beside each numeric field.
- Click the calculate button to view the result card.
- Review permeability, gradient, velocity, and interpretation values.
- Use the CSV and PDF buttons to export your result summary.
Frequently Asked Questions
1. What does coefficient of permeability represent?
It measures how easily water moves through soil or porous material. Larger values show easier flow. Smaller values indicate tighter soil structure and stronger resistance to seepage.
2. When should I use the constant head method?
Use it for coarse-grained soils with relatively higher permeability. Sands and gravels commonly suit constant head testing because measurable discharge occurs within a practical time range.
3. When is the falling head method better?
Use it for finer soils with lower permeability. Silts and some clays produce slower flow, so falling head observations usually provide more stable and measurable readings.
4. Why does the calculator ask for porosity?
Porosity is optional. It lets the page estimate seepage velocity, which is often higher than Darcy velocity because actual flow occurs only through connected pore spaces.
5. Do units need to match before calculation?
No. The calculator converts entered units to SI internally. You can mix supported units safely, but the measurements must still describe the same physical test.
6. Why might field values differ from lab values?
Field structure, layering, temperature, air entrapment, sample disturbance, and incomplete saturation can change permeability. Laboratory values are useful, but they should not replace engineering judgment.
7. What does the plotted graph show?
It shows predicted discharge against hydraulic gradient using the calculated permeability and specimen area. This helps visualize how flow changes as the driving gradient increases.
8. Can this calculator replace site investigation?
No. It is a fast planning and checking tool. Final design decisions should still consider site borings, gradation data, groundwater conditions, and project-specific codes.