Earthing Grid Resistance Calculator
Estimate the resistance-to-earth of a buried earthing/grounding grid from soil resistivity, grid area, conductor length and ground rods, using the IEEE Std 80 Sverak equation — checked against a configurable target resistance.
Input
Typical ranges (IEEE Std 142): wet organic soil 10–50, moist clay/loam 100, sandy clay 150–300, dry sand or gravel 1000–2500, bedrock 3000–10000 Ω·m. This usually dominates the result — measure it on site where possible.
Area enclosed by the outer perimeter of the buried grid conductors.
Total length of horizontal conductor buried in the grid (all runs and cross-connections summed).
Depth of the grid conductors below grade — commonly 0.3–1.0 m.
Number of vertical ground rods driven at the grid and bonded to it.
Driven length of each rod. Ignored when Ground Rods is 0.
Design ceiling to check the estimate against — commonly 25 Ω for a single equipment ground (NEC 250.53), tightened to around 1–5 Ω for a substation or critical facility (IEEE Std 80).
Output
| Quantity | Value |
|---|---|
| No data yet | |
Guides
A buried earthing (grounding) grid's resistance to earth depends far more on soil resistivity than on how the grid itself is built — but grid area, total conductor length, burial depth and any ground rods still all move the number. This calculator applies the IEEE Std 80 Sverak equation, the standard single-equation screening formula engineers reach for before running a full multi-layer soil model, and checks the result against a target resistance so the output reads as a pass/fail rather than a lone unlabeled figure.
How to use it
- Enter the soil resistivity (Ω·m). This is the dominant variable — measure it on site with a four-point (Wenner) test where possible, rather than guessing.
- Enter the grid area — the area enclosed by the outer perimeter of the buried conductors — and the total conductor length buried in the grid.
- Set the burial depth of the grid conductors below grade.
- Add any ground rods, with the driven length per rod. Leave rods at 0 for a grid with no rods.
- Set a target resistance to compare the estimate against — 25 Ω is a common ceiling for a single equipment ground, tightened to around 1–5 Ω for a substation or critical facility.
The tool reports the estimated earthing resistance, the formula breakdown, and whether the design meets the target.
The formula
Rg = ρ × [ 1/L + 1/√(20A) × (1 + 1/(1 + h√(20/A))) ]ρ— soil resistivity (Ω·m)L— total buried conductor length: grid conductor + (rod count × rod length)A— area enclosed by the grid (m²)h— burial depth (m)
This is the Sverak simplification of the Schwarz equation from IEEE Std 80, valid for a grid buried in reasonably uniform soil.
Why does soil resistivity matter so much?
ρ is a linear multiplier on the whole result — doubling it roughly doubles the resistance, while doubling the grid area only shrinks the second term by about 1/√2. A resistivity measurement (or at minimum a good regional estimate) matters more than any refinement to the grid geometry.
What target resistance should I use?
It depends on the installation and the applicable code. 25 Ω is a widely cited NEC ceiling for a single made electrode; utility substations and critical facilities are typically designed well below that, often 1–5 Ω, following an IEEE Std 80 study. Check the standard that governs your installation rather than treating either number as universal.
Is this enough to sign off a grounding design?
No. This is a single-layer-soil screening estimate. Soil layering, seasonal moisture changes, actual fault current magnitude, and touch/step voltage at the surface all need a proper grounding study — this tool is for sizing a first-pass design before that study, not replacing it.
Privacy
Everything is calculated in your browser. No site details are sent anywhere.
Related tools
To estimate the fault current the grounding system needs to safely dissipate, use the Short Circuit Current Estimator. For the transformer feeding the site, see the Transformer Sizing Calculator.
Use it from code
From 3 credits per callREST API
curl -X POST https://api.iotools.cloud/v1/tool/earthing-grid-resistance-estimator \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"soilResistivity": "100",
"gridArea": "100",
"conductorLength": "200",
"burialDepth": "0.5",
"rodCount": "4",
"rodLength": "3",
"targetResistance": "25"
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `earthing-grid-resistance-estimator` tool (Earthing Grid Resistance Calculator) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.