Ground Rod Resistance Calculator
The resistance of a driven rod to earth is dominated by soil resistivity and by rod length — and much less than people expect by rod diameter. Doubling length roughly halves resistance; doubling diameter changes it by only a few percent. That is why grounding designs add rods and depth rather than thicker rods, and why the NEC's 25-ohm threshold is met with a second electrode rather than a bigger one.
Run the numbers
The working calculator is free and needs no signup.
The formula
| ρ | Soil resistivity in ohm-metres, which varies enormously: wet clay is tens, dry sand or rock is thousands |
| L | Rod length in metres |
| d | Rod diameter in metres |
| R | Resistance to earth in ohms |
Worked example
A 3 m rod, 16 mm diameter, in 100 ohm-metre soil:
R = [100 / (2π × 3)] × [ln(4 × 3 / 0.016) − 1]
R = 5.31 × (6.62 − 1) = 29.8 ohms
Above the 25-ohm figure, so a second electrode is required. Doubling the rod to 6 m brings it to roughly 17 ohms; doubling the diameter instead would barely move it.
Which standard governs this
NEC 250.53(A)(2) requires a supplemental electrode where a single rod exceeds 25 ohms. IEEE Std 81 covers measurement of earth resistivity and ground resistance; IEEE Std 80 covers substation grounding design.
What this calculation does not account for
Dwight's formula assumes uniform soil, which real ground is not — resistivity varies with depth, moisture and season, and a summer measurement can be far better than the same rod in a dry autumn. It models one rod: multiple rods interact, and their combined resistance is higher than simple parallel arithmetic suggests unless they are spaced at least one rod length apart.
Common mistakes
Treating a calculation as a substitute for measurement, when the code requirement is about measured resistance. Specifying a larger diameter to hit a resistance target. Spacing multiple rods too closely, so their shells of influence overlap and the array performs worse than expected.