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Isolated Ground Rod Earth Resistance Calculator (Dwight's Formula)

Calculate a single driven ground rod's earth resistance using Dwight's formula.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Estimated ground rod resistance

40.43 Ω

Check your inputs

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.

At the values currently entered, the estimated ground rod resistance works out to 40.4 Ω. Confidence is moderate: the method is sound, but real materials and site conditions vary. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

Add the equipment this sizes

This result is a specification — 40.428 Ω — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • 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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Frequently asked questions

What does Dwight's formula calculate?
It estimates the earth resistance of 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's diameter. It's a well-established, widely-cited formula (per IEEE Std 142) for single ground rod resistance.
Why does rod diameter matter so little compared to rod length?
Diameter only appears inside the logarithm term (ln(8L/d)), so doubling the diameter has a small effect on the result, while rod length appears both inside the logarithm and as a direct divisor — driving a longer rod is a much more effective way to lower resistance than using a thicker one.
Is this calculated value good enough for a code compliance test?
No — this is an estimate only. Soil resistivity varies significantly with moisture, temperature, and composition, so it should be field-measured (e.g. via a Wenner four-pin test) for your actual site. NEC 250.53(A)(2) requires a single rod to test at 25Ω or less (or a second rod must be added), and that determination must come from an actual field measurement, not a calculated estimate.