Earthing Resistance Calculator (Earth Rod)
By Santaji GadepatilinUpdated October 27, 2026
An earth electrode is only as good as its resistance to the ground around it. Lower resistance lets fault current flow safely and keeps touch voltages low. Soil resistivity and rod length matter most, as shown in this earthing resistance calculation and the overview of TN, TT and IT earthing systems.
How to use the earthing resistance calculator
Enter the soil resistivity (measured with a Wenner four-pin test, or estimated from the table), and the length and diameter of the rod or pipe. The calculator gives the resistance of a single electrode.
For more background, read Earthing in Industrial Process Plants.
The formula (Dwight)
R = ρ ÷ (2πL) × [ln(8L ÷ d) − 1]
Soil resistivity
| Soil | Typical resistivity (Ω·m) |
|---|---|
| Marshy ground, clay with moisture | 2–50 |
| Loam and garden soil | 5–100 |
| Clay, sandy clay | 20–200 |
| Sand and gravel | 100–1,000 |
| Dry sand | 500–5,000 |
| Rock | 1,000–10,000+ |
Resistivity rises sharply when soil dries out or freezes, so measure in the dry season where possible.
Single rod resistance (16 mm rod)
| Length | 50 Ω·m | 100 Ω·m | 300 Ω·m |
|---|---|---|---|
| 1.2 m | 35.8 Ω | 71.6 Ω | 214.7 Ω |
| 2 m | 23.5 Ω | 47.0 Ω | 141.0 Ω |
| 3 m | 16.7 Ω | 33.5 Ω | 100.5 Ω |
| 4.5 m | 11.9 Ω | 23.8 Ω | 71.3 Ω |
| 6 m | 9.3 Ω | 18.6 Ω | 55.8 Ω |
Multiple rods
Rods in parallel do not divide resistance perfectly because their resistance areas overlap. Spaced at least one rod length apart, two rods give about 55–60% of one rod’s resistance, three about 40%, and four about 33%. Closer spacing gives less improvement.
Testing
Measure earth resistance with the fall-of-potential (three-pole) method, with the current spike at least 5–10 times the electrode length away. Clamp-on testers work only where there is a parallel return path.
Frequently asked questions
What is the formula for earth rod resistance?
R = ρ ÷ (2πL) × (ln(8L ÷ d) − 1), the Dwight formula used in IEEE 142 and IS 3043, where ρ is soil resistivity, L the rod length and d its diameter.
What earth resistance is acceptable?
Common targets are 1 Ω or less for substations, under 5 Ω for industrial and electrical equipment earthing, and up to 10 Ω for lightning protection. Follow your local standard.
Does a thicker rod lower resistance?
Only slightly. Doubling the diameter reduces resistance by about 10–15%. Length and soil resistivity matter far more.
How do I reduce earth resistance?
Drive longer rods, add more rods spaced at least their own length apart, or treat the soil around the electrode with backfill compound.