Earthing Resistance Calculator

Work out earthing resistance instantly with clear inputs, formula shown and shareable results.

About Earthing Resistance Calculator

For a driven rod, resistance is (rho / 2 pi L)(ln(8L/d) - 1). Length matters far more than diameter because it appears outside the logarithm, so doubling rod length nearly halves resistance while doubling diameter barely helps. Parallel rods must be spaced at least their own length apart to avoid overlapping resistance shells.

Formula

Rod electrode resistance

R = (rho / (2 pi L)) x (ln(8L/d) - 1); parallel rods give less than proportional benefit

Worked example

With the default values below, Earthing Resistance Calculator returns Earth electrode resistance of 26.92. Change any input above and every figure here is recalculated in your browser.

Inputs used

Rod length: 2.4 m
Rod diameter: 16 mm
Soil resistivity: 100 Ω·m
Number of rods in parallel: 2 rods

Results

Earth electrode resistance: 26.92 Ω
Single rod resistance: 40.39 Ω
Typical target: 10.0 Ω
Assessment: Too high - add rods, lengthen or treat the soil

Step by step

1Values used
2Rod electrode resistance
3Earth electrode resistance= 26.92 Ω
4Single rod resistance= 40.39 Ω
5Typical target= 10.0 Ω
6Assessment= Too high - add rods, lengthen or treat the soil

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

Why does rod diameter hardly matter?

It appears only inside a logarithm, so doubling it reduces resistance by a few percent. Driving deeper or adding rods is far more effective.

Why is parallel efficiency less than 100%?

Adjacent rods share the same soil volume, so their resistance areas overlap. Spacing them at least one rod length apart limits the loss to around 20-25%.