Grounding Calculator
Estimate the resistance-to-ground of a vertical rod electrode using the classic Dwight equation, then check it against the NEC 25 Ω single-rod rule. Soil type, rod length, and rod diameter are the three inputs that matter — and soil type matters most.
Electrode & Soil
The Dwight Equation
For a single vertical rod driven below the surface:
R = (ρ / 2πL) × (ln(8L / d) − 1)
ρ is soil resistivity in ohm-meters, L the buried rod length in meters, and d the rod diameter in meters. The equation shows why length and soil dominate: resistance falls roughly linearly with L (and faster with the log factor), while doubling rod diameter only trims the ln term slightly. A standard 8 ft × 5/8" rod in clay/loam (ρ = 60) lands near 24 Ω — right at the 25 Ω code threshold.
Resistance by Soil Type
| Soil Type | Resistivity | 1 Rod (8 ft 5/8") | 2 Rods, 16 ft apart |
|---|
Single-rod figures from the Dwight equation for an 8 ft × 5/8-inch rod. Two-rod figures assume parallel rods spaced ≥16 ft apart. NEC 250.53(A)(2) governs rod requirements.
How to Use This Calculator (3 Steps)
Case Study
Frequently Asked Questions
What is the grounding resistance formula?
R = (ρ/2πL) × (ln(8L/r) − 1) for a single rod. ρ is soil resistivity, L length, r radius.
How many ground rods do I need?
NEC 250.53(A)(2): one 8 ft rod suffices if it tests at 25 Ω or less; otherwise add a second.
What is good soil resistivity?
Wet clay 5-30 Ω·m, loam ~60, sand 200-500, rock up to 4000. Lower is better.
How do I measure ground resistance?
Fall-of-potential or clamp-on tester per IEEE 81. This calculator is a design estimate, not a test.
Is 25 ohms to ground good?
It's the NEC single-rod minimum. Substations and telecom often demand 1-5 Ω.