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Earthing Resistance Calculator
Estimate the resistance of a driven rod or ring earth electrode from soil resistivity and geometry, before a real measurement is taken.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Math & Science
Earthing Resistance Calculator
Frontend preview — no upload or external service.
Estimated resistance
A 2.4 m rod of 16 mm diameter in 100 Ω·m soil gives roughly 40 Ω — doubling the length to 4.8 m brings it to about 23 Ω, not 20.
How the Earthing Resistance Calculator works
- Measure or estimate soil resistivity; it varies by an order of magnitude between wet clay and dry sand.
- Enter the electrode geometry.
- Treat the result as a design estimate and confirm with a real earth resistance measurement on site.
The method
Rod resistance depends mostly on soil resistivity and rod length, and only weakly on diameter, because resistance is dominated by the soil immediately surrounding the electrode.
R ≈ (ρ / 2πL) x (ln(8L/d) - 1)
With ρ = 100 Ω·m, L = 2.4 m and d = 0.016 m the result is about 40 Ω. Doubling the diameter changes it by only a few percent — length and soil are what matter.
FAQ
Why does a thicker rod barely help?
Because the diameter appears inside a logarithm. Driving deeper, adding rods, or improving soil conditions all beat a fatter electrode by a wide margin.
Does soil moisture change the result?
Enormously. Resistivity can rise several times over in a dry summer, so an installation measured after rain may not meet its target in August. Design with margin.
Can I rely on this instead of measuring?
No. It is a design estimate from an idealised model of uniform soil. Regulations require a measured value, and real soil is layered and variable.
How we compare
| Feature | Online Tool Store | A graphing calculator | A stats package |
|---|---|---|---|
| Shows the standard formula | ✓ | ✗ | Sometimes |
| Compares rod length options | ✓ | Manual | ✓ |
| No instrument needed | ✓ | ✗ | ✗ |
| Replaces a site measurement | ✗ | ✗ | ✗ |
Earthing Resistance Calculator is a design estimate on idealised uniform soil — the compliance figure always comes from a measurement on site.