How is this different from the Wire Capacitance Calculator?

The Wire Capacitance Calculator handles one wire above a ground plane. This calculator handles two wires side by side. Different geometry and different formula. Use both when a wire pair runs above a ground plane.

Does this work for twisted pair cable?

Approximately. Twisting averages the capacitance over the twist pitch. For a rough estimate use the nominal wire separation. Cable manufacturers provide measured values on data sheets, typically 40 to 60 pF per metre for Cat5 and Cat6.

What about wires of different diameters?

The formula assumes identical wires. For unequal diameters use the geometric mean of the two radii as an approximation. For significantly different diameters a numerical field solver is more accurate.

How do I reduce wire-to-wire capacitance?

Increase separation, use thinner conductors, switch to lower permittivity insulation, or route wires perpendicular rather than parallel. Crossing at 90 degrees virtually eliminates capacitive coupling.

What is the capacitance of standard Cat5 cable?

About 52 pF per metre per pair as specified in TIA-568. This measured value is higher than a simple two-wire calculation because other pairs and the jacket add dielectric material.

Can closely-spaced wires have very high capacitance?

Yes. As separation approaches wire diameter the capacitance spikes toward infinity because the geometry approaches a parallel plate capacitor with zero gap. Insulation thickness prevents contact in practice.

Capacitance Between Wires Calculator

Capacitance Between Two Parallel Conductors

d Wire Diameter

D Centre-to-Centre Separation

L Wire Length (optional — for total capacitance)

ε_r Relative Permittivity (1 = air, 2.3 = PE)

ε_r

Wire-to-Wire Capacitance

Capacitance / metre
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Capacitance / cm

Capacitance / ft

Impedance Z0
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Wire Diameter

Separation

D/d Ratio

Permittivity

Two Parallel Wires

Two identical conductors running parallel form a distributed capacitor. The capacitance depends on the ratio of separation to wire diameter (D/d). Closer wires or thicker conductors increase capacitance.

Wire 1, Wire 2 — Two identical parallel conductors (cross-section shown). Diameter d, radius r = d/2.
D — Centre-to-centre distance between the two wires.
E-field — Electric field lines running between the two conductors. Their density determines the capacitance.
ε_r — Relative permittivity of the medium between the wires (1.0 for air, 2.3 for polyethylene insulation).