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Ground Rod Resistance Calculator

Estimate resistance-to-earth of a driven ground rod from soil resistivity, rod length and diameter, using Dwight's formula.

Single Rod Resistance

Soil resistivity presets are order-of-magnitude only (moisture, compaction, and mineral content routinely swing real resistivity by 10× within one "soil type") — a site resistivity test is the real number.
The commonly cited 25-ohm figure (NEC 250.53(A)(2)) is a pass/fail test for whether a single rod/pipe/plate electrode needs a supplemental electrode — it's enforceable rule text (an Exception), not just an informational note, and it doesn't apply once you're already using multiple electrodes. It's also not a general "the grounding system must measure 25Ω" requirement. Always confirm against the NEC edition your jurisdiction has adopted.

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.

The formula

R = [ρ / (2πL)] × [ln(4L/d) − 1]
ρSoil resistivity in ohm-meters, which varies enormously: wet clay is tens, dry sand or rock is thousands
LRod length in meters
dRod diameter in meters
RResistance to earth in ohms

Worked example

A 3 m rod, 16 mm diameter, in 100 ohm-meter 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.

This is a screening estimate. It is here to get you to the right order of magnitude and the right conversation — not to replace a stamped calculation by a qualified engineer.

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.

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