The formula
where ρ is soil resistivity (Ω·m), L is rod length, and d is rod diameter, both in metres. Soil resistivity varies hugely with moisture, composition and season — wet or marshy ground can be as low as 10-50 Ω·m, while dry sandy soil or rock can run into the thousands.
When one rod isn't enough
Most codes target an overall system resistance of roughly 5-25Ω depending on the installation, and a single rod in poor soil often can't reach that on its own. The usual next steps are adding more rods (spaced far enough apart to avoid overlapping resistance zones), using a ring or grid electrode instead of a single deep rod, or soil treatment to lower resistivity around the electrode. Note this formula is for one isolated rod — it doesn't account for the mutual resistance effect between multiple closely spaced rods.
Frequently asked questions
What's a good target resistance for an earthing system?
It depends on the installation and governing code, but ≤5Ω is often targeted for critical/sensitive equipment (like substations or data centres), while ≤25Ω is a commonly cited general target for smaller installations. Check your local code for the specific requirement.
Why does soil type matter so much?
Soil resistivity is the dominant variable in the formula and can span two to three orders of magnitude — from ~10 Ω·m in wet clay to over 1,000 Ω·m in dry rock — so the same rod can give wildly different results depending purely on ground conditions.
Can I just add more rods to hit my target resistance?
Generally yes, though not in simple linear proportion — rods spaced too close together develop overlapping resistance zones and interfere with each other (mutual resistance), reducing the benefit of each additional rod. This single-rod formula doesn't model that interaction; a full ring/grid or multi-rod design should account for spacing.