Free Earthing Tool
Soil Resistivity Calculator
Convert a Wenner four-pin test reading into apparent soil resistivity in ohm-metres. Enter equal pin spacing and the resistance displayed by the tester.
Engineering check required: This result is indicative only. Verify it with a licensed electrical engineer or electrical consultant.
Apparent soil resistivity (ρ)Planning result, not a design certificate. This simplified equation assumes equally spaced, shallow test pins and approximately level, locally uniform ground. Record the test layout and conditions, repeat in more than one direction and have earthing design decisions reviewed by a qualified engineer.
What the Wenner four-pin method measures
The Wenner method places four electrodes in a straight line at equal spacing a. A soil-resistivity tester drives current through the two outer pins and measures voltage between the inner pins; their ratio is the displayed resistance R. IS 3043:2018 describes this four-electrode arrangement in Clause 7.3.2. The result is apparent resistivity because real ground is rarely a single uniform material: changing the spacing changes the volume and approximate depth of soil sampled.
Resistivity is an input to estimating electrode or grid resistance, so testing before layout and procurement is more defensible than selecting a value from a generic soil table. Measurements at several spacings can reveal variation with depth and help an earthing designer decide whether the site needs further investigation. The calculator does not predict the resistance of a finished earth electrode.
Soil resistivity formula in ohm-metres
For equally spaced pins whose driven depth is small compared with their spacing, IS 3043 gives the familiar Wenner approximation ρ = 2πaR. In plain terms, the tester has measured voltage divided by current in ohms; the geometric factor 2πa, in metres, converts that reading to ohm-metres. This page applies only that shallow-pin form. IS 3043 also provides a depth-corrected expression when pin depth is appreciable, so do not use the simple calculator for that geometry.
Worked example
Suppose four pins are equally spaced at 5 m and the tester reads 12.5 Ω. Substitute the recorded values:
Report the result with the spacing and test conditions, not merely as “the site resistivity”. A series of readings at other spacings may differ because each arrangement samples a different effective volume of layered ground.
Common Wenner test pitfalls
Keep all four pins collinear and equally spaced, and measure spacing consistently between adjacent pins. The standard’s simplified equation treats pin depth as negligible relative to spacing; if that condition is not met, use the full depth correction rather than silently accepting bias. Avoid buried metallic services and other conductors that can distort current paths, and repeat the traverse in another direction when practical.
Moisture and temperature change soil resistivity. IS 3043 notes that seasonal variation can be considerable and that resistivity rises rapidly when moisture content becomes very low; a convenient wet-season reading may therefore be optimistic for year-round design. Log recent weather, ground condition and date, then plan around representative adverse conditions under competent engineering supervision.
Engineering references checked
- IS 3043:2018, Code of Practice for Earthing, Clauses 7.1–7.3.2 and Figure 1: soil variation, four-electrode test arrangement, shallow-pin equation and depth correction. Obtain the licensed standard from the Bureau of Indian Standards.
- IEEE Std 81-2012, Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System: independent engineering reference for the Wenner method and interpretation of apparent resistivity. See the IEEE Standards record.