Earthing Resistance Calculator

Calculate ground resistance from rod count, length, diameter, spacing and soil resistivity.

Input Parameters

Results

Single Rod Resistance — Ω
Utilization Factor η —
Total Parallel Resistance — Ω
Reduction vs Single Rod —

Formula Used

R_single = (ρ / 2πL) × ln(4L/d)
R_total = R_single / (N × η)

Quick Reference

Wet Clay
~10 Ω·m
Moist Loam
~50 Ω·m
Dry Sand
~500 Ω·m

Frequently Asked Questions

How do you calculate earthing resistance for multiple rods?

First compute single rod resistance using R = (ρ / 2πL) × ln(4L/d), where ρ is soil resistivity, L is rod length and d is rod diameter. Then for N rods in parallel, apply R_total = R_single / (N × η), where η is the utilization factor (typically 0.6–0.9) that accounts for mutual interference depending on rod spacing.

What is the ideal spacing between ground rods?

The ideal spacing between earthing rods is at least equal to the rod length (S ≥ L). This minimizes mutual coupling and maximizes the utilization factor. Greater spacing improves parallel efficiency but requires more land area.

How does soil resistivity affect ground resistance?

Soil resistivity (ρ) is directly proportional to ground resistance. Higher resistivity soils such as dry sand or rock increase resistance, while moist clay and loam lower it. Measured ρ values typically range from 10 Ω·m for wet clay to over 10,000 Ω·m for dry rock.