What formula does this calculator use?

It uses the standard coaxial cable formula: Z0 = (138 / sqrt(Er)) × log10(D / d), where D is the inner diameter of the outer conductor (shield), d is the outer diameter of the inner conductor, and Er is the relative permittivity of the dielectric. This formula assumes a lossless, ideal coaxial line.

What is the dielectric constant for common coax insulators?

Common values: Polyethylene (PE) ≈ 2.25, used in RG-58/59/6; PTFE (Teflon) ≈ 2.07, used in low-loss RF cables; Foam PE ≈ 1.45–1.6, used in low-loss cables; Air ≈ 1.0 (ideal, not practical). Use 2.25 for most standard coax unless the datasheet specifies otherwise.

Why are 50 ohms and 75 ohms the standard impedances?

50 ohms is a compromise between maximum power handling (~30 ohms) and minimum loss (~77 ohms in air), chosen for RF and test equipment. 75 ohms minimizes signal loss and is used for cable TV, satellite, and video distribution where signal integrity over long runs is more important than power handling.

What is velocity factor?

Velocity factor (VF) is the speed of signal propagation in the cable as a fraction of the speed of light in vacuum. It equals 1 / sqrt(Er). For polyethylene (Er = 2.25), VF ≈ 0.667, meaning signals travel at 66.7% of the speed of light. VF matters when cutting cables to a specific electrical length for antennas or phasing harnesses.

What units should I use for the diameters?

Both diameters must be in the same unit — the calculator uses millimeters (mm). Only the ratio D/d matters for the impedance calculation, so the unit does not affect the result as long as both values use the same unit. If you have inch measurements, multiply by 25.4 to convert to mm.

Does conductor material affect impedance?

No — the characteristic impedance depends only on the geometry (diameter ratio D/d) and the dielectric constant. Conductor material (copper, silver, aluminum) affects attenuation and resistive losses but not Z0. Use the impedance formula for impedance; use the manufacturer loss table for attenuation per meter.

Coax Impedance Calculator

Enter inner conductor diameter, outer conductor diameter, and dielectric constant to instantly calculate coaxial cable characteristic impedance.

Cable Parameters

All diameters in millimeters (mm)

Common Presets

Inner conductor outer diameter (d)

Outer conductor inner diameter (D)

Relative permittivity / dielectric constant (Er)

dimensionless

Polyethylene = 2.25 | PTFE = 2.07 | Foam PE ≈ 1.5 | Air = 1.0

Results

Enter cable parameters and click Calculate.

Standard Coax Reference

Type	Z0 (Ω)	d (mm)	D (mm)
RG-58	50	0.90	2.95
RG-59	75	0.64	3.71
RG-6	75	1.024	4.572
LMR-400	50	2.74	7.24
RG-178	50	0.51	1.676

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Summary

This tool calculates the characteristic impedance (Z0) of a coaxial cable using the standard formula Z0 = (138 / sqrt(Er)) × log10(D/d), where D is the outer conductor inner diameter, d is the inner conductor outer diameter, and Er is the relative permittivity of the dielectric. Characteristic impedance is critical for RF and microwave systems — mismatched impedance causes signal reflections, power loss, and standing waves. Common coax standards are 50 ohms (RF/test equipment) and 75 ohms (cable TV/video).

How it works

Enter the inner conductor outer diameter (d) in millimeters.
Enter the outer conductor inner diameter (D) in millimeters — this is the inner surface of the shield.
Enter the relative permittivity (dielectric constant, Er) of the insulating material — 2.3 for PTFE, 2.25 for polyethylene, 1.0 for air.
Click Calculate to apply the coaxial impedance formula.
The result shows characteristic impedance Z0 in ohms along with propagation velocity as a percentage of the speed of light.
Use the preset buttons to load typical coax configurations like RG-58 or RG-6 instantly.

Use cases

Design custom coaxial cables for 50-ohm RF systems and test equipment.
Verify dimensions of RG-58, RG-59, RG-6, and other standard coax types.
Calculate propagation velocity for time-domain reflectometry (TDR) analysis.
Match coax impedance to antenna feed points in amateur radio builds.
Select dielectric material to achieve a target impedance with fixed conductor diameters.
Reverse-engineer conductor dimensions needed for a specific impedance target.
Evaluate impedance of home-made or salvaged coaxial cables before installation.
Cross-check manufacturer datasheets during RF circuit prototyping.

Frequently Asked Questions

Last updated: 2026-05-29 · Reviewed by Nham Vu