Earthing Electrode Resistance Calculator
Estimate the resistance to earth of a single driven earth rod using the standard Dwight / IEEE 142 formula. Enter the soil resistivity, the rod length and its diameter, and optionally a number of parallel rods, to get the electrode resistance in ohms.
Enter Values
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How to use this calculator
- Enter the soil resistivity ρ in ohm-metres — ideally measured on site (it ranges from a few Ω·m for damp clay to hundreds for dry sand or rock).
- Enter the rod length L in metres and the rod diameter d in millimetres.
- Optionally enter the number of rods in parallel to see the ideal widely-spaced combined resistance.
How it works
The single-rod resistance uses the Dwight formula R = ρ/(2π·L)·(ln(4L/d) − 1), with the rod diameter converted from millimetres to metres. Longer rods and lower-resistivity soil give lower resistance; diameter has only a weak (logarithmic) effect. For n rods in parallel the tool applies the ideal estimate R_n ≈ R/n, which assumes the rods are spaced far enough apart (at least a rod-length) that their resistance areas don't overlap.
Worked example
Worked example. For ρ = 100 Ω·m, L = 2.4 m and d = 16 mm (0.016 m): ρ/(2π·L) = 100/(2π × 2.4) = 6.6315, and ln(4 × 2.4 / 0.016) = ln(600) = 6.3969. So R = 6.6315 × (6.3969 − 1) = 6.6315 × 5.3969 = 35.79 Ω.
Common mistakes
- Entering the rod diameter in metres when the field expects millimetres (16 mm, not 0.016) — the tool does the conversion for you.
- Assuming four rods give exactly a quarter of the resistance — R/n only holds for widely-spaced rods; closely-spaced rods give more than R/n.
- Using a guessed soil resistivity — it dominates the result and varies hugely with soil, moisture and temperature, so measure it on site.
Frequently asked questions
Why does adding rods not divide the resistance exactly by n?
The R/n rule assumes each rod sits in its own undisturbed patch of soil. When rods are close together their resistance areas overlap and they partly 'compete' for the same soil, so the combined resistance is higher than R/n. Space rods at least one rod-length apart and apply a combining factor for a real design value.
Does rod diameter make much difference?
Not much. Diameter only appears inside a logarithm, so doubling the diameter changes the resistance by a small amount. Rod length and soil resistivity are the two factors that really move the result — driving the rod deeper into lower-resistivity soil is far more effective than making it thicker.
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