An earth pit (ground electrode) gives fault and lightning currents a safe path into the soil. Its resistance to the mass of earth determines how well it does that job. For a single driven rod the classic estimate is the Dwight formula, which combines soil resistivity with the rod's length and diameter. Because resistance scales inversely with length but only logarithmically with diameter, driving deeper — into moister, lower-resistivity soil — is the most effective way to bring the value down.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: H.B. Dwight (1936); IEEE 80, BS 7430 earthing references.
Safety notice. Earthing is life-safety equipment. This single-rod estimate is for planning only; the installed resistance must be measured (fall-of-potential method) and the design verified against your local standard by a qualified engineer. See our disclaimer.
The Dwight formula
Where R is resistance in ohms, ρ is soil resistivity in Ω·m, L is the rod length in metres and d the rod diameter in metres (so a 16 mm rod is d = 0.016 m). The dominant term is ρ: halve the soil resistivity and you halve the resistance. For an array of N rods spaced well apart, the resistance falls towards R/N, but field overlap means real efficiency is about 80–90%.
Worked example — bringing 33 Ω down to target
Scenario: A single 3 m, 16 mm rod in 100 Ω·m loam measures about 33 Ω, but the design needs ≤ 10 Ω.
Dividing the ideal way, 33 / 10 = 3.3, so at least four rods are needed (allowing for ~85% efficiency). Four rods spaced 3 m apart give roughly 33 / 4 / 0.85 ≈ 9.7 Ω, just under target. Alternatively, a single 6 m rod nearly halves the single-rod value to about 18 Ω — deeper is efficient, but a multi-rod array also helps current sharing during a lightning strike.
For the protection design that drives the earthing target, see the lightning protection calculator.
Frequently Asked Questions
Dwight formula: R = (ρ ÷ (2πL)) × (ln(8L ÷ d) − 1), with ρ in Ω·m, L and d in the same units. ρ = 100 Ω·m, L = 3 m, d = 16 mm gives ≈33 Ω.
It varies: ~1 Ω for large installations/substations, under 5 Ω for many commercial systems, under 10–25 Ω for small installations and lightning protection. Follow IEC 60364 / IEEE 80 / BS 7430.
It is directly proportional and dominates. Clay/loam 10–100 Ω·m, sand/gravel 200–1000, rock thousands. Dry or frozen soil multiplies resistance several times.
Yes, but not in direct proportion — fields overlap. Space rods at least one rod-length apart (ideally 2×); real parallel efficiency is ~80–90%, so plan a little more than R ÷ N.
Resistance falls ~inversely with length but only logarithmically with diameter, so deeper beats fatter — and deeper reaches moister, lower-resistivity soil.