Twisted pair impedance & crosstalk calculator

Estimate the differential characteristic impedance of a twisted pair from wire diameter, center spacing and the dielectric — the check for RS-485, CAN and Ethernet.

Twisted pair geometry

Conductor diameter

Wire diameter, mm (~0.51 for 24 AWG).

Center-to-center spacing

Distance between the two wire centers, mm.

Dielectric constant (εr)

~2.3 for PE twisted-pair insulation, ~4 for PVC.

Results

Differential impedance: 109.6Ω
Capacitance: 46.2pF/m
Inductance: 0.555µH/m
Velocity factor: 0.659
Propagation delay: 5.1ns/m

Typical target: RS-485 / RS-422 and CAN (100–120 Ω). RS-485/RS-422 buses are usually 100–120 Ω, CAN 120 Ω, USB 2.0 differential 90 Ω and Ethernet 100 Ω.

Quick reference

Two-wire line estimate at common pairs — 22 AWG at ~1.3 mm lands near 100 Ω.

Pair	Impedance
22 AWG (0.643 mm) at 1.3 mm, εr 2.3	110.4 Ω
24 AWG (0.511 mm) at 1.0 mm, εr 2.3	107.8 Ω
26 AWG (0.405 mm) at 0.9 mm, εr 2.3	117.9 Ω
28 AWG (0.32 mm) at 0.7 mm, εr 2.3	116.7 Ω

About this estimate

Real twisted-pair impedance depends on the insulation geometry and twist rate — treat the figure above as an estimate, not a spec. Differential impedance keeps common-mode noise from turning into signal error, which is why RS-485 and CAN runs are paired as twisted pairs.

Browse related parts

Browse related parts:

RS-485 ribbon cable wire

Frequently asked questions

What impedance should a twisted pair be for RS-485?

RS-485 and RS-422 buses are usually 100–120 Ω, CAN runs 120 Ω, USB 2.0 differential 90 Ω and Ethernet 100 Ω. The tool estimates impedance from Z ≈ (276 / √εr) × log10(2s/d) and flags whether your geometry lands inside the 100–120 Ω window.

How do I calculate twisted pair characteristic impedance?

For a closely spaced two-wire line the estimate is Z ≈ (276 / √εr) × log10(2s/d), where d is the conductor diameter, s the centre-to-centre spacing and εr the insulation dielectric constant (about 2.3 for PE, 4 for PVC). Capacitance and inductance follow the same line model, and velocity factor is vf = 1/√εr.

Why does 24 AWG at 1.0 mm spacing give about 100 Ω?

Because the ratio 2s/d sits where the log term balances the 276 / √εr constant. The built-in quick reference lists 24 AWG (0.511 mm) at 1.0 mm and εr 2.3 at about 108 Ω, and 22 AWG at 1.3 mm near 110 Ω. Real impedance still depends on insulation geometry and twist rate, so treat the figure as an estimate rather than a spec.

How it works

The differential impedance of a closely spaced two-wire line is estimated as Z ≈ (276 / √εr) × log₁₀(2s/d), where d is the conductor diameter, s the center-to-center spacing and εr the dielectric constant of the insulation. Capacitance and inductance follow the same line model, and the velocity factor comes from vf = 1/√εr. Twisting tightens the coupling slightly, so the exact value still depends on the insulation geometry and twist rate. IC Source Direct provides this tool for reference only.

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