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How strongly the surrounding soil resists electrical current flow.
Soil resistivity varies widely with moisture content, temperature, and composition. For an actual site, measure it directly (e.g. with a Wenner four-pin test) rather than assuming a textbook value.
The length of the ground rod actually driven into the earth.
A standard driven ground rod is commonly 2.4 m (8 ft) long; longer rods or multiple rods reduce resistance further in high-resistivity soil.
The rod's outer diameter.
Common ground rod diameters are 5/8 in (0.0159 m) and 3/4 in (0.0191 m).
Estimated ground rod resistance
39.9 Ω
Soil resistivity varies significantly with moisture, temperature, and composition — use a field-measured value (e.g. Wenner four-pin test) for your actual site rather than an assumed value. NEC 250.53(A)(2) requires a single rod to test at 25Ω or less, or a second rod must be added; this calculator estimates the resistance, it does not replace an actual field measurement.
They open the calculator with your figures already in it
Isolated Ground Rod Earth Resistance Calculator (Dwight's Formula): 39.89 Ω — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Dwight's formula for a single driven ground rod: R = (ρ ÷ (2×π×L)) × (ln(8L÷d) − 1), where ρ is soil resistivity, L is the rod's driven length, and d is the rod diameter — a well-established, widely-cited formula for estimating single ground rod resistance (IEEE Std 142)
Inputs used
- Soil Resistivity ρ (Ω·m)
- 100
- Rod driven length
- 8 ft
- Rod Diameter d (m, e.g. 0.0159 for 5/8 in)
- 0.02
Confidence note: Soil resistivity varies significantly with moisture, temperature, and composition — use a field-measured value (e.g. Wenner four-pin test) for your actual site rather than an assumed value. NEC 250.53(A)(2) requires a single rod to test at 25Ω or less, or a second rod must be added; this calculator estimates the resistance, it does not replace an actual field measurement.
What this calculation does not cover
- Dwight's formula assumes one uniform soil for the full depth of the rod, and real ground is layered. A wet clay skin over dry sand, or two feet of topsoil over rock, cannot be reduced to a single resistivity — and if the rod refuses on rock at 1.2 m then the length in the formula is 1.2 m, not the 2.4 m of steel that was bought. A Wenner traverse at several pin spacings reveals the profile; one number conceals it, and the error runs to a factor rather than a percentage.
- Resistivity is seasonal, and the shallow soil moves the most. It climbs steeply as ground dries and again as it freezes, where it can rise by an order of magnitude, so the upper metre — the part that dries and freezes — contributes least at exactly the times it is needed. A rod that measures 20 Ω after spring rain can be well past the 25 Ω threshold in February or in a drought, which is why driving below the frost line does far more than diameter ever will.
- One isolated rod. Add a second and the resistance does not halve, because the two current fields overlap — set them closer than the driven length apart and the pair behaves more like a single larger electrode than like two in parallel, which is the reason a minimum separation exists at all. Nor does this see the rest of the electrode system: a concrete-encased electrode, a metal water service or bonded building steel sits in parallel with the rod and usually dominates whatever a fall-of-potential or clamp-on test actually reads.
- A low rod resistance does not clear a fault. On a grounded AC system the fault current returns to the source along the equipment grounding conductor, and the earth is explicitly not permitted to serve as that path — a flawless 5 Ω rod will not operate a breaker. The rod is there for lightning, for static and for holding the system's voltage reference, so ohms in the ground are no substitute for a continuous low-impedance bonded return.
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This result is a specification — 39.9 Ω — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Dwight's formula for a single driven ground rod: R = (ρ ÷ (2×π×L)) × (ln(8L÷d) − 1), where ρ is soil resistivity, L is the rod's driven length, and d is the rod diameter — a well-established, widely-cited formula for estimating single ground rod resistance (IEEE Std 142)
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