ELECTRONICS · AC CIRCUIT
Capacitive Reactance Calculator (Xc)
Compute capacitive reactance from frequency & capacitance, solve for the unknown variable, combine series/parallel capacitors, and analyze series RC impedance + phase angle.
Xc = 1 / (2πfC)
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| Frequency | Xc |
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Ideal capacitor model: ESR, ESL, and leakage current are ignored. At very high frequencies, results may deviate from real capacitors.
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Z = √(R² + Xc²) · φ = arctan(−Xc/R)
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Negative phase angle = current leads voltage (characteristic of capacitive circuits). At R = 0, φ = −90°.
Worked examples
1. Xc from f and C
C = 10 µF, f = 50 Hz → Xc = 1/(2π·50·10⁻⁵) = 318.31 Ω.
2. Xc at radio frequency
C = 100 nF, f = 1 kHz → Xc = 1,591.55 Ω (1.59155 kΩ).
3. Series & parallel capacitors
100 nF in series with 220 nF → Ctotal = 68.75 nF. 100 nF in parallel with 220 nF → Ctotal = 320 nF.
4. Series RC impedance
R = 1 kΩ, C = 100 nF, f = 1 kHz → Xc = 1,591.55 Ω, Z = √(1000² + 1591.55²) = 1,879.64 Ω, φ = −57.86°.
Frequently asked questions
What is capacitive reactance (Xc)?
Capacitive reactance is the opposition a capacitor presents to alternating current (AC), measured in ohms (Ω). Its formula is Xc = 1/(2πfC), where f is frequency in hertz and C is capacitance in farads. Unlike a resistor, capacitive reactance depends on frequency.
Why does capacitive reactance fall as frequency rises?
Because f is in the denominator of Xc = 1/(2πfC), Xc is inversely proportional to frequency. Example: a 10 µF capacitor at 50 Hz has Xc = 318.31 Ω, but at 500 Hz its Xc is only 31.83 Ω. This is why capacitors are used as high-pass filters.
How do you calculate capacitors in series?
Use 1/Ctotal = 1/C1 + 1/C2 + ... Example: 100 nF in series with 220 nF gives Ctotal = 68.75 nF — always smaller than the smallest capacitor. Series capacitors also raise the total voltage rating of the circuit.
How do you calculate capacitors in parallel?
Total parallel capacitance is the direct sum: Ctotal = C1 + C2 + ... Example: 100 nF in parallel with 220 nF gives Ctotal = 320 nF. Parallel connection is used to increase capacitance.
What is series RC circuit impedance?
The total impedance Z of a series resistor-capacitor circuit is Z = √(R² + Xc²), with phase angle φ = arctan(−Xc/R). Example: R = 1 kΩ, C = 100 nF, f = 1 kHz → Xc = 1,591.55 Ω, Z = 1,879.64 Ω, and φ = −57.86°.
What does a negative phase angle mean in an RC circuit?
A negative phase angle means current leads voltage (voltage lags current). This is characteristic of capacitive circuits — the opposite of inductive circuits, whose phase angle is positive. For a pure capacitor (R = 0), the phase angle is exactly −90°.
Does this calculator account for capacitor ESR?
No. This calculator uses the ideal capacitor model — Equivalent Series Resistance (ESR), parasitic inductance (ESL), and leakage current are ignored. At very high frequencies or with old electrolytic capacitors, real ESR can change results significantly.
Which units does this calculator support?
Capacitance: pF, nF, µF, mF, F. Frequency: Hz, kHz, MHz. Resistance/reactance/impedance: Ω, kΩ, MΩ. Results are automatically shown in the most readable unit.
Why does a capacitor block DC current?
DC current has frequency f = 0. Plug it into Xc = 1/(2πfC): the denominator is zero, so Xc approaches infinity — the capacitor behaves like an open circuit for DC (once fully charged).
When should I use series vs parallel capacitor mode in this calculator?
Use series mode when you need a smaller capacitance than available, or a higher voltage rating. Use parallel mode when you need a larger capacitance — for example combining capacitors to reach 320 nF from 100 nF + 220 nF.
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