Capacitance Between Wires Calculator

Capacitance Between Wires Calculator
Capacitance Between Two Parallel Conductors
d Wire Diameter
D Centre-to-Centre Separation
L Wire Length (optional — for total capacitance)
εr Relative Permittivity (1 = air, 2.3 = PE)
εr
Wire-to-Wire Capacitance
Capacitance / metre
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Capacitance / cm
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Capacitance / ft
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Impedance Z0
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Wire Diameter
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Separation
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D/d Ratio
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Permittivity
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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 − r D (centre-to-centre) E-field d = 2r C/m = πε₀εr / acosh(D/2r)
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.
εr — Relative permittivity of the medium between the wires (1.0 for air, 2.3 for polyethylene insulation).

Capacitance Between Wires Calculator

Two parallel conductors running side by side form a distributed capacitor along their length. This wire-to-wire capacitance determines crosstalk between adjacent signals, limits bandwidth in long cable runs, sets the characteristic impedance of twin-wire transmission lines, and adds capacitive load to driver circuits. Enter wire diameter and centre-to-centre separation — the calculator returns capacitance per metre, total capacitance, and characteristic impedance.

The Formula

C/m = πε0εr / acosh(D / 2r) — capacitance per metre
Z0 = (120 / √εr) × acosh(D / 2r) — characteristic impedance

D = centre-to-centre separation
r = wire radius (d/2)
ε0 = 8.854 × 10−12 F/m
εr = relative permittivity (1 for air)

The acosh (inverse hyperbolic cosine) form is exact for any ratio of separation to diameter — it does not require the common simplification that D must be much larger than d. Accurate even when wires are barely separated, which is exactly where simplified formulas break down.

Wire-to-Wire vs Wire-to-Ground

The Wire Capacitance Calculator uses C = 2πε0εr / ln(2h/r) for a single wire above a ground plane. This calculator uses the twin-wire formula because the geometry is symmetric — two equal conductors facing each other rather than one conductor facing an infinite plane. Different geometry, different formula, different results. The D/d ratio (separation divided by diameter) is the key number that controls everything: larger ratio means lower capacitance and higher impedance.

Calculator Inputs

d (Wire Diameter) — conductor diameter in mm, inches, or AWG (0–40). For insulated wire, use the conductor diameter, not the insulation outer diameter. Both wires are assumed identical.

D (Centre-to-Centre Separation) — distance between wire centres in mm, cm, or inches. Must be greater than d (wires cannot overlap). For ribbon cable: the pitch (typically 1.27 mm). For speaker cable: the centre-to-centre spacing inside the jacket.

L (Wire Length) — optional. Parallel run length. Multiplied by C/m for total capacitance.

εr (Relative Permittivity) — dielectric constant of the medium between the wires. Air = 1. Polyethylene = 2.3. PVC ≈ 3.4. Higher εr increases capacitance linearly and decreases impedance by √εr.

Worked Example — Twin Lead in Air

1 mm diameter wires, 5 mm centre-to-centre, 1 m long, air.

r = 0.5 mm, D = 5 mm
D/2r = 5 / 1.0 = 5.0
acosh(5.0) = 2.292

C/m = π × 8.854×10−12 / 2.292 = 12.13 pF/m
Total: 12.13 pF for 1 m

Z0 = 120 × 2.292 = 275 Ω

12 pF per metre, 275 Ω impedance. This is a basic open twin-wire transmission line — the high impedance and low capacitance make it suitable for balanced RF feedlines where 300 Ω is the target (classic TV antenna twin-lead uses a similar geometry with polyethylene webbing).

Worked Example — Speaker Cable

AWG 16 conductor (d = 1.291 mm), 3.5 mm separation, 5 m long, polyethylene insulation (εr = 2.3).

r = 0.646 mm, D = 3.5 mm
D/2r = 3.5 / 1.291 = 2.712
acosh(2.712) = 1.649

C/m = π × 8.854×10−12 × 2.3 / 1.649 = 38.8 pF/m
Total: 38.8 × 5 = 194 pF for 5 m

Z0 = (120 / √2.3) × 1.649 = 79.1 × 1.649 = 130 Ω

194 pF over a 5 m run. At audio frequencies (20 Hz–20 kHz) this capacitance is irrelevant — the impedance of 194 pF at 20 kHz is ~41 kΩ, far above any speaker impedance. But in long speaker cable runs (50+ metres) used in distributed PA systems, the total capacitance can reach nanofarads and starts to roll off high frequencies with amplifiers that have high output impedance.

Worked Example — Ribbon Cable

0.5 mm conductors at 1.27 mm pitch (standard 0.050-inch IDC ribbon), 300 mm long, air.

r = 0.25 mm, D = 1.27 mm
D/2r = 1.27 / 0.5 = 2.54
acosh(2.54) = 1.566

C/m = π × 8.854×10−12 / 1.566 = 17.76 pF/m
Total: 17.76 × 0.3 = 5.33 pF for 300 mm

Z0 = 120 × 1.566 = 188 Ω

5.3 pF between adjacent conductors in a 30 cm ribbon cable. This is the crosstalk coupling capacitance — a fast edge on one signal capacitively couples a voltage spike onto the neighbouring signal. At 10 MHz with a 1 V signal swing, the coupled current through 5.3 pF is I = C × dV/dt ≈ 5.3 pF × 10 V/µs = 53 µA. Into a 50 Ω termination, that is about 2.7 mV of crosstalk. Tolerable for digital logic, problematic for precision analog signals. For how this capacitance interacts with the circuit’s time constant, see the RC Time Constant Calculator.

Dielectric Effect on Capacitance and Impedance

InsulationεrCapacitance MultiplierImpedance Multiplier
Air1.01.0×1.0×
PTFE (Teflon)2.12.1×0.69×
Polyethylene2.32.3×0.66×
PVC3.43.4×0.54×
Silicone rubber3.23.2×0.56×

PVC insulation more than triples the wire-to-wire capacitance compared to air at the same geometry. This is why high-quality cables use polyethylene or PTFE — lower permittivity means lower capacitance and higher (closer to air-spaced) impedance.

Practical Impact

Crosstalk

Capacitive coupling between adjacent wires injects noise from one signal onto another. The coupled voltage is proportional to capacitance, signal edge rate (dV/dt), and the victim circuit’s impedance. Reduce crosstalk by increasing wire separation (larger D), using ground wires between signal wires, or switching to shielded cable.

Bandwidth Limitation

Wire-to-wire capacitance forms a low-pass filter with the driver’s output impedance. A 1 nF cable capacitance with a 100 Ω driver gives τ = 100 ns, rolling off above ~1.6 MHz. Long cable runs in process control (4–20 mA loops, fieldbus) must account for this capacitive loading.

Transmission Line Impedance

The characteristic impedance Z0 of a twin-wire line comes directly from this geometry. 300 Ω twin-lead uses specific wire diameter and spacing in polyethylene webbing to hit the target impedance. The calculator shows Z0 so you can verify or design balanced transmission lines. For the fundamental V = IR relationship behind impedance matching, see the Ohm’s Law Calculator.

Capacitive Load on Drivers

Every driver must charge and discharge the total cable capacitance on every signal transition. Higher capacitance means more dynamic current (I = C × dV/dt), slower edges, and higher power consumption. This matters for long UART, SPI, and I2C buses.

Frequently Asked Questions

How is this different from the Wire Capacitance Calculator?
That calculator handles one wire above a ground plane (wire-to-ground capacitance). This one handles two wires side by side (wire-to-wire capacitance). Different geometry, different formula. Use both when you need the complete picture — a wire pair above a ground plane has both wire-to-wire and wire-to-ground capacitance.
Does this work for twisted pair cable?
Approximately. Twisting changes the effective separation slightly and averages the capacitance over the twist pitch. For a rough estimate, use the nominal wire separation. For precise twisted pair capacitance, cable manufacturers provide measured values on their data sheets (typically 40–60 pF/m for Cat5/Cat6).
What about wires of different diameters?
The formula assumes identical wires. For unequal diameters, use the geometric mean of the two radii: reff = √(r1 × r2). This is an approximation — for significantly different diameters, a numerical field solver gives a more accurate answer.
How do I reduce wire-to-wire capacitance?
Increase separation (D). Use thinner conductors. Switch to a lower-εr insulation (PTFE instead of PVC). Route wires perpendicular rather than parallel where possible — crossing at 90° virtually eliminates capacitive coupling.
What is the capacitance of standard Cat5 cable?
About 52 pF/m (15.8 pF/ft) per pair, as specified in TIA-568. This is the measured value including the effects of twisting, jacket, and other pairs — higher than a simple two-wire calculation because the other pairs and the jacket add additional dielectric material around the conductors.
Can closely-spaced wires have very high capacitance?
Yes. As D approaches d (wires nearly touching), acosh(D/2r) approaches zero and capacitance spikes toward infinity — because the geometry approaches a parallel-plate capacitor with zero gap. In practice, the insulation thickness prevents contact, but the capacitance of wires inside a tight cable bundle can be surprisingly high.

Last updated: March 2026