Ground Rod Resistance Calculator (Earth Electrode, Dwight)
Resistance to earth of a single vertical ground rod from the soil resistivity, rod length and diameter, and the voltage it rises to for a given current.
Results
The dashed line marks the value you entered.
What this tool does
Dwight's formula gives the resistance of a rod driven vertically into uniform soil, and the soil resistivity is by far the biggest unknown: it runs from a few tens of Ω·m in damp clay to many hundreds in dry sand or rock, and it changes with moisture and season. Length matters much more than thickness — doubling the length cuts the resistance by a bit under half, while doubling the diameter trims it by only about a tenth — so deeper or additional rods are what bring it down. This is an estimate for planning; the real value of an installed electrode has to be measured on site.
Formula
R = ρ ÷ (2πL) × (ln(4L ÷ a) − 1) (Dwight, vertical rod of length L and radius a in uniform soil)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
ps | Soil resistivity | Ω·m |
l | Length of the rod in the ground | m |
d | Diameter of the rod | mm |
i | Current flowing into the ground | A |
R | Resistance of the rod to earth | Ω |
V | Voltage of the rod above remote earth | V |
R2 | Resistance with a rod twice as long | Ω |
Worked example
- Soil resistivity100 Ω·m
- Length of the rod in the ground2.4 m
- Diameter of the rod16 mm
- Current flowing into the ground25 A
- Resistance of the rod to earth40.4 Ω
- Voltage of the rod above remote earth1010 V
- Resistance with a rod twice as long22.5 Ω
Limitations
- Electrical installations are governed by national wiring rules. Cable sizing also depends on installation method, grouping, ambient temperature and protection devices, which this calculator does not evaluate.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.