OneCalcApp LogoOneCalcAppConvert. Calculate. Simplify.

Earthing Calculator

ElectricalInstant resultsFormula shown100% in your browser

Calculate earth electrode resistance, single-rod and multi-rod grounding resistance, the earthing conductor cross-section from fault current and clearing time, and the number of rods needed to reach your own design target.

Inputs

1

Soil and electrode

These five inputs alone determine the calculated earth electrode resistance.

Drives the result more than anything else. Use a measured Wenner survey value where you have one; typical guide ranges only: clay 5–50, loam 50–200, sand 200–1000, rock >1000.

Driven depth of one electrode. Length is the strongest geometric lever — a longer rod reaches deeper, damper soil and lowers resistance roughly in proportion.

Enters the formula inside a logarithm, so a bigger rod changes resistance only slightly; it mainly buys mechanical and corrosion life.

Rods in the electrode array. Extra rods lower resistance, but never by the full 1/n because their resistance shells overlap.

Centre-to-centre distance. Spacing below one rod length increases mutual interference, which this calculator applies as an interference factor.

2

Fault conditions and conductor

Used for the earthing conductor cross-section and the design target check.

Earth-fault current the conductor must carry. Used only for the adiabatic conductor-size result, not for the electrode resistance.

Protection disconnection time. Thermal duty on the earthing conductor grows with √t, so faster clearing allows a smaller conductor.

Sets the material constant k in A = I√t/k — copper 143, aluminium 95, galvanised steel 78.

Your own design target for this installation — not a universal legal value. The calculator reports how many rods that target needs.

How to use this calculator: Earthing Calculator

Calculate earth electrode resistance, single-rod and multi-rod grounding resistance, the earthing conductor cross-section from fault current and clearing time, and the number of rods needed to reach your own design target. The example below is calculated by this page's real engine from the displayed inputs.

  1. 1Confirm that the Earthing Calculator matches the quantity or design check you need.
  2. 2Enter Soil resistivity, Rod length, and Rod diameter using the units printed beside each field.
  3. 3Select the applicable Earth conductor material options; these choices change the calculation method or factors.
  4. 4Calculate, then follow the substituted equations in the worked example and compare the result with any stated limit.
  5. 5Read the assumptions, warnings and cited references before using the result for a financial, medical or engineering decision.

Input guide and example values

Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.

InputExample valueWhy it matters
Soil resistivity100 Ω·mDrives the result more than anything else. Use a measured Wenner survey value where you have one; typical guide ranges only: clay 5–50, loam 50–200, sand 200–1000, rock >1000.
Rod length3 mDriven depth of one electrode. Length is the strongest geometric lever — a longer rod reaches deeper, damper soil and lowers resistance roughly in proportion.
Rod diameter17.2 mmEnters the formula inside a logarithm, so a bigger rod changes resistance only slightly; it mainly buys mechanical and corrosion life.
Number of rods3Rods in the electrode array. Extra rods lower resistance, but never by the full 1/n because their resistance shells overlap.
Rod spacing3 mCentre-to-centre distance. Spacing below one rod length increases mutual interference, which this calculator applies as an interference factor.
Prospective earth-fault current10000 AEarth-fault current the conductor must carry. Used only for the adiabatic conductor-size result, not for the electrode resistance.
Fault clearing time1 sProtection disconnection time. Thermal duty on the earthing conductor grows with √t, so faster clearing allows a smaller conductor.
Earth conductor materialCopper (k = 143)Sets the material constant k in A = I√t/k — copper 143, aluminium 95, galvanised steel 78.
Target earth resistance1 ΩYour own design target for this installation — not a universal legal value. The calculator reports how many rods that target needs.

Formula, derivation and worked example

This is the general electrical earthing calculator for buildings, industrial installations and equipment grounding — it works out the earth electrode resistance of a driven-rod array, the earthing conductor cross-section for the fault duty you enter, and how many rods your own resistance target would need. The resistance of a driven rod is dominated by the soil immediately around it, so length helps far more than diameter: doubling the length nearly halves the resistance while doubling the diameter changes it by only a few percent. Multiple rods must be spaced at least their own length apart, otherwise their resistance shells overlap and the group performs worse than n parallel rods.

Single rod: R = ρ / (2πL) × (ln(8L/d) − 1)
n rods: R_n = R / n × (1 + λ), λ accounts for mutual interference
Conductor size: A = I × √t / k (IEC 60364-5-54 adiabatic equation)

Substitution steps

  1. 1. Single rod
    ρ/(2πL) × (ln(8L/d) − 1)
    = 33.109 Ω
  2. 2. Spacing ratio
    s / L
    = 1
  3. 3. Group resistance
    R/n × (1 + λ)
    = 15.451 Ω
  4. 4. Conductor area
    I √t / k
    = 69.93 mm²

Computed example results

Earth resistance of the array
15.45 Ω
⚠️ above the target
Single rod resistance
33.11 Ω
Interference factor
1.4
spacing is adequate
Rods needed for the target
47 rods
for ≤ 1 Ω
Earthing conductor
69.9 mm² → use 70 mm²
k = 143
Touch voltage at fault
100,000 V
before equipotential bonding

Understanding the result

Read the main result together with supporting checks, assumptions, limits and intermediate values.

For a manual check, repeat the first equation, confirm the units and change one input at a time.

Common mistakes when using Earthing Calculator

  • Do not mix units for Soil resistivity (Ω·m), Rod length (m), Rod diameter (mm). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
  • Do not leave Earth conductor material on the default choice unless that choice matches the real scenario; the selected option can change the calculation path or factor.
  • Do not replace the displayed Single rod: R = ρ / (2πL) × (ln(8L/d) − 1) relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
  • Do not treat Earth resistance of the array = 15.45 Ω from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.

When the Earthing Calculator is useful

Earthing Calculator is designed for cases where Soil resistivity, Rod length, Rod diameter, Number of rods are known and you need Earth resistance of the array, Single rod resistance, Interference factor. The page keeps the live calculator, calculation method and worked example together so the result can be checked instead of treated as a black-box number.

Use the calculator for the scope described by its inputs and notes. The displayed method is Single rod: R = ρ / (2πL) × (ln(8L/d) − 1). If the real project or decision needs factors that are not represented here, treat the result as an estimate and add the missing checks separately.

Soil resistivity and Rod length: what changes the answer

The worked example uses Soil resistivity = 100 Ω·m, Rod length = 3 m, Rod diameter = 17.2 mm, Number of rods = 3. With those values, Earth resistance of the array is 15.45 Ω. Changing an input should be interpreted according to that field's unit, range, option and hint rather than by the number alone.

For this calculator, the main input roles are: Soil resistivity (Ω·m): Drives the result more than anything else. Use a measured Wenner survey value where you have one; typical guide ranges only: clay 5–50, loam 50–200, sand 200–1000, rock >1000. Rod length (m): Driven depth of one electrode. Length is the strongest geometric lever — a longer rod reaches deeper, damper soil and lowers resistance roughly in proportion. Rod diameter (mm): Enters the formula inside a logarithm, so a bigger rod changes resistance only slightly; it mainly buys mechanical and corrosion life. Number of rods: Rods in the electrode array. Extra rods lower resistance, but never by the full 1/n because their resistance shells overlap.

How to sanity-check a Earthing Calculator result

Start by confirming the entered values and units, then compare the substituted working with the displayed formula or calculation steps. Pay particular attention to Earth resistance of the array, because it is the first worked-example output shown by the live engine.

Finally, compare the result with the assumptions, warnings and related calculators on this page. A nearby calculator can be useful as a cross-check when it measures the same workflow from a different input or output direction.

Next logical calculator

Continue with Transformer Sizing Calculator

Transformer Sizing Calculator is directly connected from Earthing Calculator as a source-defined continuation or comparison.

Open Transformer Sizing Calculator

Standards, source trail and limitations

References show the method used. Check the current local edition, amendments and project specification before a regulated decision.

Formula

  • Single rod: R = ρ / (2πL) × (ln(8L/d) − 1)
  • n rods: R_n = R / n × (1 + λ), λ accounts for mutual interference
  • Conductor size: A = I × √t / k (IEC 60364-5-54 adiabatic equation)

This is the general electrical earthing calculator for buildings, industrial installations and equipment grounding — it works out the earth electrode resistance of a driven-rod array, the earthing conductor cross-section for the fault duty you enter, and how many rods your own resistance target would need. The resistance of a driven rod is dominated by the soil immediately around it, so length helps far more than diameter: doubling the length nearly halves the resistance while doubling the diameter changes it by only a few percent. Multiple rods must be spaced at least their own length apart, otherwise their resistance shells overlap and the group performs worse than n parallel rods.

Engineering notes

  • Everything on this page follows one methodology: the Dwight rod equation, an interference factor from the spacing-to-length ratio, and the IEC 60364-5-54 adiabatic conductor equation. The worked example below uses exactly those formulas.
  • The tool is a preliminary sizing aid. A final earthing design still needs measured site resistivity, the real protection settings, and touch/step-voltage assessment where that applies.

Approximate soil resistivity ranges (educational guidance only)

Indicative ranges for orientation. Real sites vary by an order of magnitude with moisture and season — always prefer measured resistivity for design.

Soil typeTypical resistivity (Ω·m)Notes
Marshy / wet clay5–30Lowest resistance, but can dry out or be seasonal
Clay5–50Generally favourable for driven rods
Loam / farm soil50–200Common design starting point
Sand and gravel200–1000Needs longer or additional electrodes
Rock / laterite>1000Often needs backfill treatment or alternative electrodes

Formulas explained

R = ρ / (2πL) × (ln(8L/d) − 1)

Classic Dwight expression for a vertically driven rod in uniform soil. Almost all of the resistance sits in the first few rod diameters of soil around the electrode, which is why resistivity and length dominate and diameter barely moves the answer.

ρ
soil resistivity, Ω·m
L
driven rod length, m
d
rod diameter, m

R_n = R / n × (1 + λ)

Rods in a group share the same soil volume, so the array is worse than n ideal parallel rods. The interference factor λ used here grows as the spacing-to-length ratio falls below 1 and as more rods are added.

n
number of rods
λ
interference allowance from spacing ÷ rod length

Spacing at least equal to the rod length keeps interference modest.

A = I × √t / k

Adiabatic equation of IEC 60364-5-54: the conductor is assumed to receive all the fault energy with no heat lost to its surroundings, so the required area rises with fault current and with the square root of the clearing time.

I
prospective earth-fault current, A
t
fault clearing time, s
k
material constant — copper 143, aluminium 95, steel 78

The calculator then rounds up to the next standard cross-section.

Earth electrode resistance

The resistance between the buried electrode and the general mass of earth. It sets how easily fault current returns to source and how much the electrode's potential rises during a fault. Lower is better, but it is only one part of a safe earthing system.

Single rod resistance

The resistance of one rod on its own, from ρ/(2πL)×(ln(8L/d) − 1). It is the building block for the array result and shows immediately whether the soil, not the hardware, is your limitation.

Multiple rods and spacing

Adding rods lowers resistance, but overlapping resistance shells mean the array never reaches R/n. The interference factor reported by the calculator quantifies that penalty from your spacing-to-length ratio.

Earthing conductor requirement

The minimum conductor cross-section that survives the fault thermally, A = I√t/k, rounded up to the next standard size. This is what people usually mean by an earthing cable size calculation.

Target earth resistance

The value you type in is your project's design target. The calculator divides the interference-corrected single-rod resistance by it to report the number of rods required — it does not impose a target of its own.

Understanding soil resistivity

Soil resistivity (Ω·m) is how strongly the ground opposes current. It varies with soil type, moisture, temperature, salt content, compaction and depth, and swings widely between wet and dry seasons. Generic tables are educational only — use a measured four-probe (Wenner) survey for design, and check the driest expected condition.

Solar Plant Earthing Calculator

For solar PV and solar power plant grounding: buried grid, ground potential rise and touch/step voltage assessment.

Cable Sizing Calculator

Size phase conductors on ampacity and voltage drop once the earthing conductor is fixed.

Voltage Drop Calculator

Check volt drop on the same circuit in IEC or NEC mode.

Transformer Sizing Calculator

Estimate transformer rating and the LV short-circuit current that feeds an earth-fault study.

Worked example

  1. 1Example inputs (illustrative only, not a design recommendation): soil resistivity 100 Ω·m, rod length 3 m, rod diameter 17.2 mm, 3 rods at 3 m spacing, earth-fault current 10 000 A, clearing time 1 s, copper conductor, target 1 Ω.
  2. 2Step 1 — single rod: R = 100 / (2π × 3) × (ln(8 × 3 / 0.0172) − 1) ≈ 33.5 Ω.
  3. 3Step 2 — spacing ratio: s / L = 3 / 3 = 1.0, so interference stays modest.
  4. 4Step 3 — array of 3 rods: R₃ = 33.5 / 3 × (1 + λ) ≈ 13.4 Ω with the interference factor the calculator reports.
  5. 5Step 4 — earthing conductor: A = 10 000 × √1 / 143 = 70 mm², rounded up to the next standard copper size.
  6. 6Step 5 — rods for the target: the interference-corrected single-rod value divided by the 1 Ω target shows the target is unrealistic in this soil with 3 m rods, so lower resistivity, longer electrodes or a different target is needed.

Assumptions

  • Uniform soil resistivity; real sites are layered and require a Wenner four-probe survey.
  • Rods are vertically driven, of equal length, and connected by a buried conductor.
  • The conductor result is a thermal (adiabatic) minimum only — mechanical strength, corrosion allowance and any minimum size in the applicable standard still apply.

Tips

  • Space rods at least one rod-length apart; closer spacing wastes copper.
  • Chemical earthing or bentonite backfill can cut resistance by 30–60% in rocky soil.
  • If the resistance is far off target, increase rod length before adding rods — length is the cheaper lever.

Warnings

  • There is no single earth resistance value that applies everywhere. IS 3043 quotes different values for different installation types, and local regulations, the utility and the project specification may all set their own. Enter your own target.
  • Soil resistivity rises sharply in dry seasons — design against the driest condition.
  • Touch and step voltage assessment is a separate exercise; a low electrode resistance alone does not prove a grid is safe.

Standards & references

  • IS 3043:2018
  • IEC 60364-5-54

Frequently asked questions

What is an earthing calculator?

It is an online tool that estimates the earth electrode resistance of a driven-rod grounding system from soil resistivity and rod geometry, and sizes the earthing conductor from the fault current and clearing time. This page is the general electrical earthing calculator, used for buildings, industrial installations and equipment grounding.

How do I calculate earth resistance?

For a single driven rod use R = ρ/(2πL) × (ln(8L/d) − 1), where ρ is soil resistivity in Ω·m, L the rod length in metres and d the rod diameter in metres. For several rods, divide by the number of rods and apply an interference allowance based on the spacing-to-length ratio, which this calculator does automatically.

What soil resistivity should I use in an earthing calculation?

Use a measured value from a Wenner four-probe survey at the site wherever possible, taken in the driest season you must design for. Published ranges — clay roughly 5–50 Ω·m, loam 50–200, sand 200–1000, rock above 1000 — are only educational guidance for a first estimate.

How does rod length affect earth resistance?

Strongly. Resistance is inversely proportional to length in the Dwight equation, so doubling the driven length roughly halves the resistance, and a deeper rod also reaches damper, more stable soil.

Does increasing the number of earth rods reduce resistance?

Yes, but less than proportionally. Rods share the same soil volume, so an array of n rods gives more than R/n. The calculator reports an interference factor so you can see how much of the theoretical benefit you actually get.

Why is rod spacing important?

When rods are closer than about one rod length apart their resistance shells overlap and they compete for the same current path. Wider spacing reduces this mutual interference and makes each additional rod more effective.

What is the difference between earthing and grounding?

They mean the same thing — 'earthing' is the usual term in IS and IEC practice, 'grounding' in NEC and North American practice. The calculations on this page apply to both terminologies.

How is earthing conductor size determined?

With the adiabatic equation A = I√t/k from IEC 60364-5-54, where I is the prospective earth-fault current, t the disconnection time and k a material constant — 143 for copper, 95 for aluminium and 78 for galvanised steel. The result is a thermal minimum; mechanical and corrosion requirements may demand a larger size.

Related calculators

Keep going with more electrical tools.

Transformer Sizing Calculator

Select a distribution transformer kVA rating from connected load, diversity and future spare capacity, with primary and secondary full-load currents.

Cable Sizing Calculator

Size AC power cables to IEC 60364: design current, derating for ambient temperature and grouping, ampacity selection and a voltage-drop cross-check.

Voltage Drop Calculator

Free voltage drop calculator with NEC and IEC modes: get percent drop, wire size in AWG or mm², power loss and a 3% branch / 5% total check in seconds.

AC Cable Sizing Calculator

Engineering-grade inverter-to-panel AC cable design: full load current, ampacity derating, voltage drop, I²R loss and short-circuit withstand verified against IEC, NEC and IS methodology.

DC Cable Voltage Drop Calculator

Calculate DC cable voltage drop from system voltage, current, cable length, conductor material and cable size. Get voltage drop in volts, voltage-drop percentage, receiving-end voltage, loop resistance and cable power loss.

DC Cable Size Calculator (12V / 24V / 48V)

Size low-voltage DC cable for 12 V, 24 V and 48 V systems from run length and amps — get the required mm², the matching AWG, voltage drop and a full size chart.

Related Engineering Articles

Deeper reading on the engineering behind this calculation.

Earth Resistance Calculation: Formulas, Methods and Practical Examples

Learn how to calculate earth resistance using practical formulas for electrodes, earth grids and solar plant grounding systems.

Read Article →

Earth Conductor Size Calculation: Complete Grounding Guide

Calculate the required earth conductor size using fault current, clearing time, material properties and practical solar plant grounding considerations.

Read Article →

How Many Earth Rods Are Required? Complete Earthing Calculation Guide

Understand how to estimate earth rod quantity from soil resistivity, target earth resistance, electrode resistance and practical grounding layouts.

Read Article →

Earthing System Calculation: Complete Solar Plant Grounding Guide

A complete grounding calculation workflow covering soil resistivity, earth electrodes, conductors, grid resistance and practical solar plant earthing design.

Read Article →