Free Engineering Tool
Earthing and soil
Soil Resistivity Calculator (Wenner)
Enter the probe spacing and the resistance your earth tester shows for up to four spacings. Get resistivity at each depth, the average and what the trend means for your earth electrode.
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Inputs
The formula corrects for probe depth. Enter 0 to use the simple 2π·a·R form.
Leave a spacing and reading blank to skip a row.
Readout
The formula
ρ = 4π·a·R / ( 1 + 2a/√(a² + 4b²) − a/√(a² + b²) ) → ρ ≈ 2π·a·R when b is small
Four probes go in a straight line at equal spacing a. The two outer probes inject current, the two inner ones measure the voltage, and your earth tester shows R in ohms. The result is the apparent resistivity of the soil down to roughly a depth equal to the spacing.
Why take several spacings
Wider spacing sees deeper soil. If resistivity falls as spacing grows, wetter or more conductive soil lies below, so a long or deep-driven rod works well. If it rises with spacing, the good soil is near the surface, and extra rod length helps little. Use more rods, a horizontal electrode or soil treatment instead.
Reading the band
- Below 30 Ω·m: low. Typical of wet clay and organic soil.
- 30 to 100 Ω·m: moderate. Typical of loam and mixed soils.
- 100 to 300 Ω·m: high. Typical of sandy or gravelly soil.
- Above 300 Ω·m: very high. Typical of dry sand, rock and laterite.
These bands are general guide values. Resistivity swings with moisture and season, so test in the driest period you can and never treat one number as a design value. For the full method and how Wenner compares with Schlumberger, see Wenner vs Schlumberger. Then size your electrode with the earth rod calculator.
Report form: printable earth resistance and soil resistivity report (PDF).
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