How is ground rod resistance calculated?

For a single vertical rod: R = ρ/(2πL) × ln(4L/d), where ρ is soil resistivity (Ω·m), L is rod length (m), and d is rod diameter (m). This formula assumes a uniform soil model.

What is acceptable grounding resistance?

Typical targets: <5 Ω for commercial/industrial, <25 Ω for residential (NEC), <1 Ω for substations and data centers. Requirements vary by local codes and application.

How do parallel rods reduce resistance?

Parallel rods reduce total resistance, but not proportionally due to mutual interference. For N rods spaced at distance s: R_total ≈ R_single/N × (1 + interaction factor). Rods should be spaced at least 2× rod length apart for best efficiency.

What are typical soil resistivity values?

Wet clay: 10-100 Ω·m. Loam/garden soil: 50-200 Ω·m. Sand: 200-3000 Ω·m. Gravel: 3000-10000 Ω·m. Rock: 10000+ Ω·m. Soil resistivity varies greatly with moisture, temperature, and mineral content.

How do I measure soil resistivity?

The Wenner four-pin method is most common: four equally-spaced probes driven into the ground. A current is passed through the outer probes and voltage measured across the inner probes. ρ = 2πaR, where a is probe spacing and R is measured resistance.

Free Engineering Tool

Grounding Calculator

Calculate ground rod resistance using R = ρ/(2πL) × ln(4L/d). Supports single and parallel rod configurations.

IEEE 80Single / Parallel RodsSoil Resistivity

Results

Total Ground Resistance

—

Single Rod Resistance

—

Configuration

—

Status

—

Single Rod Resistance

Soil Resistivity Reference

Soil Type	ρ (Ω·m)
Wet clay	10–100
Loam / garden soil	50–200
Sand	200–3000
Gravel	3000–10000
Rock	10000+

Grounding Calculator | Ground Rod Resistance | Free Online Tool

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Results

Single Rod Resistance

Soil Resistivity Reference