Coaxial cable impedance calculator

Work out the characteristic impedance of a coaxial cable from its dimensions and dielectric — or the dimensions for a target 50/75 Ω.

Cable dimensions

Inner conductor diameter (d)

Centre conductor, in mm

Outer conductor / dielectric diameter (D)

Measured inside the shield, in mm

Dielectric

Dielectric constant

Pick a common dielectric or enter your own constant.

Characteristic impedance

Characteristic impedance: 52.98Ω
Capacitance per metre: 91.4pF/m
Inductance per metre: 0.256µH/m
Velocity factor: 0.69
Propagation delay: 4.83ns/m

Custom impedance — between the 50 Ω and 75 Ω standards. Adjust D/d to move toward either.

The 50 Ω standard balances power handling against loss; 75 Ω minimizes loss for video. Keep D/d ≈ 2.30 for 50 Ω in air (PTFE ≈ 3.07 for 50 Ω).

Related parts

Browse related parts:

coaxial connectors RF connectors BNC connectors

Frequently asked questions

What determines the characteristic impedance of a coax cable?

Impedance is set by the ratio D/d — outer (dielectric) diameter over inner conductor diameter — and the dielectric constant: Z₀ = 138 × log₁₀(D/d) / √εr. A higher D/d ratio raises impedance; a higher εr dielectric lowers it. The absolute size of the cable does not matter, only the ratio.

What D/d ratio gives 50 Ω in air?

In air (εr = 1.0) a D/d ratio of about 2.30 gives 50 Ω, because 138 × log₁₀(2.30) ≈ 50. With PTFE (εr = 2.1) the same 50 Ω needs a higher ratio of about 3.07, which is why the calculator recomputes impedance when you change the dielectric.

Why are most coax cables 50 Ω or 75 Ω?

The 50 Ω standard balances power handling against loss and is used for RF and general-purpose systems; 75 Ω minimizes attenuation and is the video/antenna standard. The calculator shows a verdict when the result lands within 2 Ω of either standard — matching connector impedance is required to avoid reflections.

How it works

IC Source Direct sizes coax the same way the RF industry does. Characteristic impedance follows Z₀ = 138 × log₁₀(D/d) / √εr, where D is the outer (dielectric) diameter, d the inner conductor diameter and εr the dielectric constant. Capacitance and inductance per metre follow C ≈ 24.2εr / log₁₀(D/d) pF/m and L ≈ 0.46 log₁₀(D/d) µH/m, and the velocity factor is VF = 1/√εr. The ratio D/d sets the impedance; the dielectric scales it down.

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IC Source Direct provides this tool for reference only.

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