Select resistance, equivalent soil resistivity, buried length, or diameter, then enter the other three values. This is an approximate single-rod model, not a safety or compliance assessment.

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Single vertical rod, top at ground level, uniform soil. Estimate only; field testing and qualified design are required. Calculator scope: L โ‰ฅ 10D, an input guard rather than a design standard.

Related Calculators

Ground Resistance Formula

This calculator uses the common single vertical ground rod approximation in uniform soil. Select the value to solve for, then enter the other three values. This calculator limits geometry to L โ‰ฅ 10D as an input guard, not a design standard. The rod top is assumed level with the surface. The calculator converts the inputs to base units, solves the missing value, and converts the result back to the unit you selected.

R = (rho / (2 ร— pi ร— L)) ร— (ln(8 ร— L / D) - 1)
  • R = ground resistance, in ohms (ฮฉ)
  • rho = soil resistivity, in ohm-meters (ฮฉยทm)
  • L = length of the ground rod, in meters (m)
  • D = diameter of the ground rod, in meters (m)
  • ln = natural logarithm
  • pi = 3.14159…

When ground resistance is the missing value, the calculator applies the formula directly:

R = (rho / (2 ร— pi ร— L)) ร— (ln(8 ร— L / D) - 1)

When soil resistivity is the missing value, the formula is rearranged as:

rho = (2 ร— pi ร— L ร— R) / (ln(8 ร— L / D) - 1)

When rod diameter is the missing value, the calculator uses:

D = (8 ร— L) / exp(1 + (2 ร— pi ร— L ร— R) / rho)

When rod length is the missing value, the length cannot be isolated cleanly with simple algebra because L appears both inside the logarithm and in the denominator. The calculator solves this equation numerically:

0 = (rho / (2 ร— pi ร— L)) ร— (ln(8 ร— L / D) - 1) - R
  • Ground resistance mode: uses soil resistivity, rod length, and rod diameter to estimate the resistance of one vertical rod.
  • Soil resistivity mode: uses a measured or target resistance with known rod dimensions to estimate the equivalent soil resistivity.
  • Rod length mode: solves for the modeled buried length for a target resistance within the calculatorโ€™s L โ‰ฅ 10D scope with the given soil resistivity and diameter.
  • Rod diameter mode: calculates the diameter that would satisfy the target resistance for the given soil resistivity and rod length.

Soil Resistivity Input Context

Soil resistivity changes with moisture, temperature, minerals, compaction, and seasonal conditions. Soil names do not determine a reliable resistivity value. Measure local conditions and relevant depths; the categories below are qualitative context, not numerical defaults.

Soil or material condition Resistivity input Effect on ground resistance
Wet clay or marshy soil Use a local measured value Usually lower resistance
Clay or moist loam Use a local measured value Moisture and local composition matter
Loam or garden soil Use a local measured value Moderate resistance
Sand or gravel Use a local measured value Higher resistance
Dry rock or very dry soil Use a local measured value Often difficult to ground effectively

Ground Resistance Result Context

Estimated resistance General interpretation
Less than 5 ฮฉ Low numerical resistance; still does not establish safety or compliance.
5 to 25 ฮฉ An illustrative resistance band, not a universal target or safety criterion.
25 to 100 ฮฉ This band alone does not establish acceptability. Requirements depend on the installation and applicable rules.
More than 100 ฮฉ Higher numerical resistance; qualified review and field measurements are needed.

Example Calculations

Example 1: Calculate ground resistance

You have soil resistivity of 100 ฮฉยทm, a rod length of 3 m, and a rod diameter of 0.016 m.

R = (100 / (2 ร— pi ร— 3)) ร— (ln((8 ร— 3) / 0.016) - 1)
R = 33.4927 ฮฉ

The estimated ground resistance is about 33.49 ฮฉ.

Example 2: Calculate soil resistivity

You measured a ground resistance of 25 ฮฉ using a 3 m rod with a diameter of 0.016 m.

rho = (2 ร— pi ร— 3 ร— 25) / (ln((8 ร— 3) / 0.016) - 1)
rho = 74.6432 ฮฉ ร— m

The estimated soil resistivity is about 74.64 ฮฉยทm.

FAQs

What does ground resistance mean?

Ground resistance is the electrical resistance between a grounding electrode, such as a ground rod, and the surrounding earth. This low-frequency single-rod estimate does not establish touch/step safety, fault protection, or lightning impulse performance. The result depends strongly on soil resistivity and electrode geometry. The diameter-form modified Dwight equation is described in the nVent ERICO guide, page 6. Installed resistance must be checked by appropriate field testing.

Why does increasing rod length usually reduce ground resistance?

A longer rod contacts more soil and reaches deeper layers that may contain more moisture. In the formula, increasing L generally lowers the resistance because the current spreads through a larger volume of earth. The reduction is not perfectly linear, so doubling the rod length does not always cut the resistance in half.

Why does rod diameter have a smaller effect than rod length?

Rod diameter appears inside the natural logarithm term, so diameter changes affect the result more slowly than length or soil resistivity. A larger diameter can reduce resistance, but increasing rod length, adding rods, or improving electrode spacing usually has a larger effect.