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Coaxial Cable Impedance Calculator — Geometry & Dielectric
Estimate coax characteristic impedance from inner-conductor diameter, shield inner diameter and relative permittivity.
Last updated: September 16, 2026
Results
Characteristic impedance 50.05571 Ω
Diameter ratio 3.35
Formula, example and assumptions
Z0≈60/√εr · ln(D/d).
Changing the D/d geometry or dielectric shifts a coax line toward common 50 Ω or 75 Ω targets.
- Lossless TEM coax approximation with homogeneous dielectric.
- Connector transitions, conductor loss, dielectric loss and frequency dispersion are not included.
Frequently asked questions
What controls coax impedance?
Mainly the ratio of shield inner diameter to center-conductor diameter and dielectric permittivity.
Why are 50 Ω and 75 Ω common?
They are long-established compromises for RF power handling, loss and video/RF system conventions.