Earth Rod Resistance Calculator
Estimate the resistance to earth of a single vertical earth rod from soil resistivity, rod length and diameter, with the formula and a worked example.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
Use this to see roughly what a single driven rod will give in a soil of known resistivity, and how much a longer or thicker rod helps. It is a design estimate to decide whether one rod could be enough or whether you will need several rods or a grid.
Limits of this calculator
The model assumes uniform soil and one rod with no other electrodes nearby. Soil moisture and season can change the real resistance several-fold, so measure on site. Parallel rods must be spaced at least their length apart to help; use the earth rods in parallel calculator for that.
Cross-check: 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Indicative
Indicative. Estimation only; further engineering checks are required. Basis: Single rod, uniform soil.
What this calculator checks
- Single-rod resistance from soil resistivity, rod length and diameter.
What remains unverified
- Measured soil resistivity, layering and seasonal variation.
- Neighbouring electrodes, and step and touch voltage.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | IEEE 81; IEC 60364-5-54, IEEE 81-2025; IEC 60364-5-54:2011 + A1:2021 Ed 3.1 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Grounding-system field measurement and earthing arrangements are relevant. The calculator uses a theoretical single-rod approximation in uniform soil; it is not a substitute for soil measurements or site testing. |
| Cross-check test cases | 6 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Indicative |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Soil resistivity (Ω·m) | 100 |
| Rod length driven in soil | 3 m |
| Rod diameter | 16 mm |
- ln(8L/d) = ln(1,500) = 7.3132
- R = 100 / (2π × 3) × (7.3132 − 1) = 33.493 Ω
Assumptions
- Uniform soil
- Single rod, no nearby electrodes
Measure on site. Soil moisture and season can change resistance several-fold; parallel rods must be spaced at least their length apart.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Soil resistivity (Ω·m) | (number) | greater than 0 | |
| Rod length driven in soil | m | greater than 0 m | |
| Rod diameter | mm | greater than 0 mm |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Earth resistance | Ω | Shown to 2 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
Typical values
| Soil resistivity (Ω·m) | Earth resistance (Ω) |
|---|---|
| 30 | 10.05 |
| 50 | 16.75 |
| 100 | 33.49 |
| 200 | 66.99 |
| 500 | 167.46 |
| 1,000 | 334.93 |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEEE Std 80 (safety in AC substation grounding)
- IEC 60364-5-54 (earthing arrangements and protective conductors)
Soil resistivity varies with season and depth; use measured values and a site-specific design for any installation. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter soil resistivity (Ω·m), rod length driven in soil and rod diameter. Change the unit of a value with its selector and the result stays the same.
Does a longer rod or a thicker rod help more?
Length helps much more. The resistance falls roughly in proportion to the length, while the diameter enters only through a logarithm, so a thicker rod changes it very little.
What soil resistivity should I use?
Use a measured value from the site, for example from a four-pin test. Soil resistivity varies widely with moisture, temperature and soil type, so a handbook figure is only a rough guide.
Is the calculated value what an earth tester will read?
Not exactly. Real soil is layered and changes with the season. Treat the calculation as a design estimate and verify the installed electrode by measurement.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Earthing Formulas: Earth Rod and Fault Loop: Earthing formulas for the resistance of a vertical earth rod, rods in parallel
- Earth Electrode Spacing Formulas: Earth electrode spacing formulas: two rods in parallel, the interaction factor
More Earthing and Grounding calculatorsAll engineering calculatorsEarth Rod Resistance in Elecmetric