Earth Rod Resistance Calculator

About this calculator

The Dwight formula calculates the electrical resistance to earth of a single driven earth (ground) rod as a function of the soil's resistivity (how strongly the surrounding earth resists current flow, measured in ohm-metres and highly dependent on soil moisture and composition), the rod's driven length, and its diameter — resistance decreases as the rod is driven deeper (with diminishing returns) but is far less sensitive to the rod's diameter, which is why adding length or adding parallel rods is the standard way to lower resistance, not switching to a fatter rod.

Electricians and electrical engineers calculate this when designing an earthing (grounding) system for a building, because most electrical codes specify a target maximum earth resistance (commonly 25 ohms or lower for many installations, tighter for sensitive equipment or lightning protection systems) that a single rod often can't meet in dry, sandy, or rocky soil — meaning the calculation tells you whether one rod suffices or how many parallel rods, spaced appropriately, are needed to hit the target.

This calculator takes your soil resistivity, rod length and rod diameter and returns the resistance of a single rod using the Dwight formula, so you can check it against your earthing target.