Calculate a conductor's resistance from its material, length, and thickness — before any current ever flows through it.
How to Use the Resistance Calculator
Pick copper, aluminum, or Custom for the conductor material, enter its length, then supply the cross-section either as a diameter or a direct area — whichever matches what you already know. Resistance recalculates on every change.
Every material resists the flow of electric current to some degree, and that resistance isn't a property of a specific wire or bar — it's a property of the substance itself, called resistivity, combined with how much of it current has to travel through. One equation ties the three together:
R = ρ × L ÷ A
A = π × (d ÷ 2)² (when entering diameter instead of area)ρ is resistivity in ohm-meters, L is length in meters, and A is cross-sectional area in square meters. Copper's resistivity sits at about 1.724 × 10⁻⁸ Ω·m and aluminum's at roughly 2.65 × 10⁻⁸ Ω·m, both standard handbook figures at room temperature.
Ohm's Law vs. This Calculator
This site's Ohm's Law Calculator also produces a resistance figure, and it's worth being clear about how the two differ. Ohm's Law backs resistance out of a live circuit — measure voltage and current with a meter, divide one by the other, and whatever component sits between those two probe points reveals its resistance indirectly, current already flowing through it. This calculator works the opposite direction: it derives resistance from a conductor's own geometry and material, before any current ever flows, purely from what the thing is physically made of and shaped like. One is a measurement technique; the other is a design calculation.
Length and area pull in opposite directions on the result. A longer conductor means more material for current to fight through, so resistance rises linearly with length — double the length, double the resistance. A fatter conductor gives current more room to spread out, so resistance falls as area grows, and grows especially fast as diameter shrinks, since area scales with the square of diameter. Thin, long wires are exactly the combination that maximizes resistance for a given material; short, thick bars minimize it.