By Ariel Balter, Ph.D.
Updated Mar 24, 2022

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In electronics, conductance quantifies how readily a circuit element allows current to flow when a voltage is applied. Denoted by the symbol G, conductance is simply the reciprocal of resistance (R) and is measured in siemens (S). It depends on the element’s shape, size, and the material’s conductivity (σ).

Conductance from Resistance

If a component’s resistance is 1.25 × 10³ Ω, its conductance is calculated as:

G = 1/R

Thus, G = 1 / (1.25 × 10³ Ω) = 8 × 10⁻⁴ S (note: the correct reciprocal value is 0.0008 S).

Conductance from Current and Voltage

When both current (I) and voltage (V) are known, Ohm’s law can be rearranged to find conductance directly:

V = I·R  →  G = I/V

For example, a 5 V supply driving 0.30 A yields:

G = 0.30 A / 5 V = 0.06 S

Conductance from Material Conductivity

For a cylindrical conductor of radius r and length L, conductance is determined by:

G = (A·σ)/L  =  (πr²·σ)/L

Example: A round iron wire with r = 0.001 m, L = 0.1 m, and σ = 1.03 × 10⁷ S/m has:

Cross‑sectional area A = π(0.001 m)² = 3.14 × 10⁻⁶ m².

Conductance: G = (3.14 × 10⁻⁶ m² × 1.03 × 10⁷ S/m) / 0.1 m ≈ 324 S.

These formulas provide a quick, reliable way to assess how efficiently a material conducts electricity.