Twisted Pair Impedance Calculator
A pair of parallel round conductors forms a transmission line whose characteristic impedance is fixed by the conductor diameter, the centre-to-centre spacing, and the dielectric surrounding the wires. This applies to twisted pairs, ribbon cable pairs, and open two-wire feeders. This calculator uses the exact inverse hyperbolic cosine formula for two-wire impedance, taking the conductor diameter, the spacing, and the effective dielectric constant, and returns the characteristic impedance along with the spacing-to-diameter ratio.
Two-wire impedance formula
ratio = spacing D / diameter d
Z0 = (120 / sqrt(er)) * arccosh(D / d)
arccosh(x) = ln( x + sqrt(x^2 - 1) )
Impedance in air uses er = 1
The inverse hyperbolic cosine form is exact for round conductors at any spacing greater than the diameter. The 120 ohm constant comes from the impedance of free space divided by pi. Dividing by the square root of the dielectric constant accounts for the slower wave in the insulation.
Twisted pair facts
- Cat 5e and Cat 6 data cabling is specified at 100 ohms.
- Bare wires in air use an effective dielectric constant of 1.
- Wider spacing for a given diameter raises the impedance.
- The spacing must exceed the diameter for the conductors not to touch.
- Twisting improves common-mode noise rejection more than it changes impedance.
Twisted pair impedance: frequently asked questions
How is two-wire line impedance calculated?
The characteristic impedance of two parallel round conductors is Z0 = (120 / sqrt(effective dielectric constant)) times the inverse hyperbolic cosine of the centre-to-centre spacing divided by the conductor diameter. For spacing much larger than diameter this approximates (120 / sqrt(er)) times the natural log of (2 times spacing over diameter).
What effective dielectric constant should I use?
For bare wires in air the value is 1. For insulated twisted pairs the effective dielectric constant lies between 1 and the insulation's relative permittivity, because the field is partly in air and partly in plastic. Typical twisted-pair cables behave around 1.5 to 2.5. This calculator exposes it as an editable input.
Why does twisting affect impedance?
Twisting mainly improves noise rejection by making the two conductors share interference equally. The characteristic impedance is set chiefly by the conductor diameter, spacing, and surrounding dielectric. The twist tightens the effective spacing slightly and helps keep it uniform along the cable.
What is a typical twisted-pair impedance?
Category data cabling such as Cat 5e and Cat 6 is specified at 100 ohms. Some telephone and instrumentation pairs are around 120 ohms. The exact figure depends on geometry and dielectric, which is why a calculator from the physical dimensions is useful for custom or unusual cables.
Does this include skin effect or frequency-dependent loss?
No. This computes the geometric characteristic impedance, which is largely frequency independent at high frequency for a low-loss line. Conductor loss from skin effect and dielectric loss are separate effects that attenuate the signal but do not strongly change the characteristic impedance.
Official sources
- NIST: Physical Measurement Laboratory: transmission line parameters.
- IEEE: IEEE standards for cabling and transmission lines.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.