Free Engineering Tool
Earthing and soil
Earth Rod Resistance Calculator
Enter soil resistivity and the rod size. Get the single-rod resistance, the effect of a longer rod, and a first estimate of how many rods reach your target.
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Readout
The formula
R = ρ / (2π·L) × ( ln(8L / d) − 1 )
This is the standard Dwight expression for one vertical rod of length L and diameter d driven into uniform soil of resistivity ρ. It assumes the rod is much longer than it is wide (L/d above about 20) and the soil is the same all the way down.
What the numbers tell you
- Length matters, diameter barely does. Doubling the length cuts resistance by roughly 40 percent. Doubling the diameter cuts it by only about 10 percent.
- Parallel rods: the ideal result is R / n, but only when rods are spaced far apart. At spacing close to one rod length the real value is noticeably higher, so treat the minimum rod count as a starting point and allow 20 to 40 percent more.
- Always confirm on site. Real soil is layered, so measure the finished earth with an earth tester using the fall-of-potential method. See how earth resistance testing works and how low is low enough.
This is an estimate for planning, not a design certificate. Seasonal moisture, rod coating, backfill and nearby buried metalwork all change the result.
Report form: printable earth resistance and soil resistivity report (PDF).
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