The resistance of a uniform wire is R = ρL/A, where ρ (rho) is the resistivity of the material in ohm-metres, L is the length in metres, and A is the cross-sectional area in square metres. Longer wires have more resistance, and thicker wires (larger area) have less. The resistivity values used here are for metals at about 20 °C.

Resistance is proportional to length and inversely proportional to cross-sectional area. Doubling the length doubles the resistance because charge carriers travel through twice as much material. Halving the area doubles the resistance because there is half as much room for the current to flow. This is why power cables use thick conductors over long runs to keep losses low.

Among common metals, silver has the lowest resistivity (about 1.59 × 10⁻⁸ Ω·m), followed by copper (1.68 × 10⁻⁸), gold (2.44 × 10⁻⁸), aluminum (2.65 × 10⁻⁸), and iron (9.71 × 10⁻⁸). Copper is the usual choice for wiring because it combines very low resistivity with reasonable cost. Aluminum is lighter and cheaper, so it is used for overhead transmission lines despite its higher resistivity.

Yes. For metals, resistivity increases with temperature, so a hot wire has a higher resistance than a cold one. The resistivity values used in this calculator are quoted at roughly 20 °C. For precise work at other temperatures you would apply a temperature coefficient of resistance, but the room-temperature figures are accurate enough for most everyday estimates.