🔌 Cable Impedance Calculator
Enter your cable’s conductor dimensions and dielectric constant to calculate its characteristic impedance.
What this calculator does
Characteristic impedance describes how a transmission line — like a coaxial or twin-lead cable — resists changes in voltage and current as a signal travels along it. Matching this impedance to your equipment avoids signal reflections and loss.
How the calculation works
For coaxial cable, impedance is Z₀ = (138 / √εr) × log₁₀(D/d), where D is the outer conductor’s inner diameter, d is the inner conductor’s diameter, and εr is the dielectric’s relative permittivity. For twin-lead (parallel wire) cable, it’s Z₀ = (276 / √εr) × log₁₀(2D/d), where D is the spacing between the two wires and d is the wire diameter.
These are the standard textbook formulas for uniform transmission lines; real cables can vary slightly due to manufacturing tolerances and non-ideal dielectric materials.