Earth Rod Resistance Calculator

How it is calculated

The resistance of one vertical rod follows the Dwight (IEEE 142) formula: R = ρ / (2π L) × (ln(8L / d) − 1). Here ρ is soil resistivity in Ω·m, L is the driven length in metres and d is the rod diameter, entered in millimetres and converted to metres. The tool then divides by your target: rods = ceil(R / target), never fewer than 1, and shows R / rods as the resistance of that many rods. That last figure is an ideal parallel value that assumes the rods are at least one rod length apart; closer rods do worse. A blank target defaults to 5 Ω. Because the formula only holds for slender rods, the tool gives no result when length ÷ diameter is below 10 (a 3 m rod must be at most 300 mm thick), or when any value is zero or negative. Resistivity changes with moisture and season, so a measurement on site is the final check.

Common mistakes

• Entering the diameter in centimetres or metres. It is in millimetres: a 16 mm rod is 16. Typing 1600 for a 1.6 m figure would be rejected as a rod far too fat for its length.
• Using a resistivity from a wet spell for a year-round design. The value changes with moisture and soil type, so use a figure that fits the driest likely conditions.
• Expecting a fatter rod to fix a high result. Diameter sits inside a logarithm and has little effect; more length or more rods lowers the resistance far more.
• Reading the multi-rod figure as a guarantee. It is the ideal parallel value; rods driven closer than about one rod length apart share the same soil, and the real result is higher.
• Putting an ohm value in the resistivity field. That field is soil resistivity in Ω·m, and the target in Ω is a different box. A target of 0 or less gives no result.