Twisted Pair Impedance Calculator

Twisted Pair Impedance Calculator determines characteristic impedance from wire diameter, wire spacing, and dielectric constants, applying a standard transmission-line formula.

Find Twisted-Pair Cable Impedance From Wire Spacing and Diameter

This tool calculates the characteristic impedance of a twisted-pair cable from conductor diameter, center-to-center spacing, and the dielectric surrounding the wires. Cable, interconnect, and hardware engineers use it to check whether a given wire geometry meets a target impedance — for Ethernet, RS-485, USB, or a custom link — before committing to a cable spec.

Entering Wire Geometry and Dielectric

Enter wire diameter (mm), center-to-center spacing (mm), relative dielectric constant of the surrounding medium (εr), and cable length (inches). The calculator returns characteristic impedance in ohms, along with propagation delay, velocity factor, and distributed inductance and capacitance per unit length.

How Twisted-Pair Impedance Is Calculated

The calculator applies the parallel-wire transmission line formula documented in Johnson and Graham’s engineering reference High-Speed Digital Design: A Handbook of Black Magic (pp. 186–187):

$$Z_0 = \frac{120}{\sqrt{\varepsilon_r}} \times \ln\left(\frac{2s}{d}\right)$$

where $s$ is the center-to-center spacing between the two conductors, $d$ is the conductor diameter, and $\varepsilon_r$ is the effective relative permittivity of the material surrounding the wires.

From $Z_0$, propagation velocity follows directly from the dielectric, $v_p = c / \sqrt{\varepsilon_r}$, and the distributed inductance and capacitance per unit length follow from $L’ = Z_0 / v_p$ and $C’ = 1 / (Z_0 \cdot v_p)$.

Common input mistake: entering the insulation’s bulk dielectric constant straight off a datasheet instead of an effective εr. A twisted pair’s field lines pass through both the wire insulation and the surrounding air, so the effective permittivity is normally lower than the insulation material’s bulk value — using the raw datasheet figure understates the true impedance.

Nuance worth knowing: because $Z_0$ scales with $\ln(2s/d)$, doubling or even quadrupling the spacing only moves impedance by tens of ohms. Per Johnson and Graham, it’s genuinely difficult to build an open wire-pair line outside roughly a 70–120 Ω range — which is why unrelated cabling standards (100 Ω Ethernet, 120 Ω RS-485, 90 Ω USB) all converge on nearby values rather than spanning it.

Wire Spacing and Diameter in Cross-Section

Common Twisted-Pair Impedance Targets

Common Questions About Twisted-Pair Impedance

What’s a typical characteristic impedance for a twisted pair?

Most unshielded pairs land between roughly 70 and 120 Ω, since impedance grows only logarithmically with wire spacing. Common standards target specific values within that band: 90 Ω for USB, 100 Ω for Ethernet, 120 Ω for RS-485.

Does twisting the wires change the calculated impedance?

This formula models straight parallel wires. Twisting maintains a roughly constant average spacing, so real impedance stays close to the prediction, but twist rate and insulation stiffness introduce small deviations the formula doesn’t capture.

Which εr value should I use for a real cable?

Use the effective relative permittivity of the medium surrounding the wires, not the bulk insulation datasheet value, since the fields also pass through air. A manufacturer’s TDR-verified spec is more accurate than an insulation datasheet alone.

Does spacing or diameter matter more for impedance?

Both enter the formula only as the ratio $2s/d$ inside a logarithm, so their individual values matter less than their ratio. Doubling spacing and diameter together leaves the calculated impedance unchanged.

Why does propagation velocity depend only on εr?

For a lossless line, velocity is set by $c / \sqrt{\varepsilon_r}$ regardless of wire geometry. Only the dielectric — not spacing or diameter — determines how fast the signal travels down the pair.