Ground Rod Calculator
This ground rod calculator estimates the resistance to earth of a driven ground rod from the soil resistivity, rod length, and rod diameter, using the Dwight formula. Enter a spacing to a second rod and the ground rod calculator also estimates the resistance of two rods in parallel, and checks the result against the NEC 25 ohm rule for a single rod.
Results
The Ground Rod Calculator is part of our Cable & Protection collection. Browse more cable & protection to size, convert, and check your work alongside this tool.
What the ground rod calculator does
A driven rod connects an electrical system to the earth. How well it does that depends mostly on the soil: the lower its resistivity, the lower the rod resistance. The ground rod calculator applies the Dwight formula for a single vertical rod, then estimates two rods in parallel from their spacing, so you can see whether a second rod brings the system below 25 ohms.
Ground rod resistance formula
The Dwight formula for a single vertical rod and a simple estimate for two rods:
ρ is soil resistivity in Ω·m, L the rod length in m, a the rod radius in m, and s the spacing between rods. The second term accounts for the rods sharing the same soil.
Ground rod calculator example
A 5/8 in (15.875 mm) rod driven 2.4 m into soil of 100 Ω·m:
One rod is above 25 Ω, so the NEC calls for a supplemental electrode; two rods at 6 ft spacing bring the estimate under 25 Ω.
How to use the ground rod calculator
Enter the soil resistivity in ohm meters, from a soil test or a typical value for your soil. Enter the driven length of the rod in meters, 2.4 m for an 8 ft rod, and its diameter in millimeters, 15.875 mm for a 5/8 in rod. To check a second rod, enter the spacing between them in meters. Press Calculate.
Reading ground rod resistance
The result is an estimate; soil moisture, temperature, and layers change the real value, so measure installed rods with a ground resistance tester. Under the NEC, a single rod, pipe, or plate electrode that does not measure 25 ohms or less must be supplemented by another electrode, and in practice many installers drive two rods at least 6 ft apart and skip the test. Spacing the rods at least as far apart as one rod length lets each work in its own soil.
When to use an earth rod resistance calculator
- Planning a grounding electrode system for a service
- Checking whether one rod is likely to meet 25 ohms
- Choosing rod length and spacing in poor soil
- Estimating ground resistance for telecom or lightning systems
- Teaching how soil resistivity drives ground resistance
Common ground rod calculator mistakes
- Using feet for length. Enter meters; 8 ft is 2.44 m.
- Using radius as diameter. Enter the full rod diameter in mm.
- Rods too close. Keep at least 6 ft apart.
- Trusting the estimate alone. Measure installed rods where it matters.
Tips for lower ground rod resistance
- Drive rods to full depth; partial driving raises resistance.
- Use two rods 6 ft or more apart as a standard practice.
- Bond all electrodes on site into one system.
Soil resistivity and ground rod resistance
Soil resistivity varies by a factor of a hundred or more. Wet clay and loam can be under 30 Ω·m, while dry sand, gravel, and rock can exceed 1,000 Ω·m. Moisture and dissolved salts carry the current, so the same site can read much higher in a dry summer or when frozen. A four pin Wenner test measures resistivity at depth before you drive rods; without one, the table values give a rough idea. In high resistivity soil, longer or deeper rods, several rods, ground rings, or concrete encased electrodes do far better than one standard rod. The Ohm law calculator shows why lower resistance matters: it sets how much voltage the rod rises to during a fault.
Ground rod spacing and multiple rods
Each rod is surrounded by a zone of soil that carries its current. Rods driven close together share that zone, so two rods side by side barely beat one. At about one rod length apart they act more independently, and the two rod estimate here drops close to half the single rod value. Beyond two or three rods, the gains shrink and a ground ring or plate becomes more practical. Remember that the grounding electrode is not what trips the breaker; the equipment grounding conductor sized with the fault current in mind does that.
Ground rod resistance chart
Single 5/8 in rod resistance by soil resistivity:
| Soil resistivity | 8 ft (2.44 m) rod | 10 ft (3.05 m) rod |
|---|---|---|
| 10 Ω·m (wet clay) | 4.0 Ω | 3.3 Ω |
| 30 Ω·m | 12.0 Ω | 9.9 Ω |
| 50 Ω·m | 20.0 Ω | 16.5 Ω |
| 100 Ω·m | 39.9 Ω | 33.1 Ω |
| 200 Ω·m | 79.8 Ω | 66.2 Ω |
| 500 Ω·m (dry sand, gravel) | 199.5 Ω | 165.4 Ω |
Ground Rod Calculator FAQ
How do you calculate ground rod resistance?
Use the Dwight formula R = ρ ÷ (2πL) × [ln(4L ÷ a) − 1]. An 8 ft, 5/8 in rod in 100 Ω·m soil gives about 40 Ω.
What is the NEC 25 ohm rule?
A single rod, pipe, or plate electrode that does not have a resistance to earth of 25 ohms or less must be supplemented by an additional electrode.
How far apart should two ground rods be?
The NEC requires at least 6 ft between rods. Spacing them at least one rod length apart, or more, gives a better result.
Does a longer rod help more than a thicker one?
Yes. Resistance falls roughly with length, while diameter appears only inside the logarithm, so doubling the diameter changes the result only slightly.