Capacitive Reactance Calculator
This Capacitive Reactance Calculator determines how strongly an ideal capacitor opposes alternating current at a selected frequency. Enter frequency and capacitance to calculate Xc in ohms and angular frequency in radians per second.
Capacitive reactance is important when selecting coupling and bypass capacitors, estimating AC impedance, checking filter behavior, and evaluating whether a capacitor presents a suitably low impedance across a signal or noise frequency range.
The calculation uses ideal capacitance. Practical designs should also account for ESR, ESL, tolerance, dielectric behavior, DC bias, package parasitics, and self-resonant frequency.
Capacitive Reactance Calculator
Calculate capacitor reactance and angular frequency from capacitance and AC frequency. Inputs are converted to hertz and farads before calculation.
AC or signal frequency applied to the capacitor.
Effective capacitance at the selected operating conditions.
Capacitive reactance (Xc)
1.592 kΩ
Xc = 1 / (2πfC)
- Reactance in ohms
- 1,591.549Ω
- Human-readable reactance
- 1.592kΩ
- Angular frequency (ω)
- 6,283.185rad/s
At the selected frequency, the ideal capacitor opposes AC current with 1.592 kΩ of reactance. Increasing either frequency or capacitance lowers this value.
Capacitor in an AC Circuit
The capacitor presents a frequency-dependent reactance between the AC source and load. Higher frequency or larger capacitance produces a lower ideal Xc.
Capacitive Reactance Formula
Frequency and capacitance appear in the denominator, so increasing either value lowers capacitive reactance.
Xc = 1 / (2πfC)ω = 2πfXc = 1 / (ωC)- Xc = Capacitive reactance magnitude in ohms (Ω)
- f = Frequency in hertz (Hz)
- C = Capacitance in farads (F)
- π = Pi, approximately 3.14159
- ω = Angular frequency in radians per second (rad/s)
How to Use This Calculator
- Enter the AC, signal, switching, or noise frequency and choose Hz, kHz, MHz, or GHz.
- Enter the effective capacitance and select pF, nF, µF, mF, or F.
- Select Calculate to obtain Xc in ohms, a human-readable engineering value, and angular frequency ω.
- Compare the ideal result with capacitor ESR, ESL, self-resonance, and circuit impedance before final component selection.
Worked Example
100 nF Capacitor at 1 kHz
f = 1 kHz = 1,000 Hz
C = 100 nF = 100 × 10⁻⁹ F
Xc = 1 / (2π × 1,000 × 100 × 10⁻⁹)
Xc ≈ 1,591.55 Ω
Xc ≈ 1.59 kΩ
ω = 2π × 1,000 ≈ 6,283 rad/s.
Engineering Notes
Reactance decreases with frequency
Doubling frequency halves ideal Xc. This behavior allows capacitors to block steady DC while passing changing AC signals more readily.
Larger capacitance lowers Xc
At the same frequency, a larger capacitor produces proportionally lower reactance and can carry more AC current for a given voltage.
Coupling capacitors need low reactance
Choose coupling capacitance so Xc is sufficiently below the surrounding circuit impedance at the lowest required signal frequency.
Bypass performance spans a frequency range
Bypass capacitors should present low impedance across relevant noise frequencies. Placement, package inductance, and parallel values influence broadband behavior.
Real capacitors include parasitics
ESR, ESL, tolerance, leakage, dielectric losses, voltage coefficient, and self-resonance make real impedance differ from ideal Xc.
Common Mistakes
- Using kHz or nF directly without converting to Hz and F.
- Entering zero frequency even though ideal Xc approaches infinity at DC.
- Treating reactance magnitude as a complete real-capacitor impedance model.
- Ignoring self-resonant frequency when evaluating high-frequency bypassing.
- Using nominal capacitance without checking tolerance and DC bias.
FAQ
What is capacitive reactance?
Capacitive reactance is the frequency-dependent opposition an ideal capacitor presents to alternating current. It is measured in ohms and represented by Xc.
How do you calculate capacitive reactance?
Use Xc = 1 / (2πfC), where frequency is in hertz and capacitance is in farads. The result is the reactance magnitude in ohms.
Why does capacitive reactance decrease with frequency?
At higher frequency, capacitor voltage reverses more rapidly and more current flows for the same voltage amplitude. Since frequency is in the formula denominator, increasing it reduces Xc.
What happens to capacitive reactance at DC?
For ideal steady-state DC, frequency is zero and capacitive reactance approaches infinity. After charging, the ideal capacitor behaves like an open circuit, although real leakage current can still flow.
Is capacitive reactance the same as impedance?
Xc is the positive magnitude of ideal capacitive reactance. Ideal capacitor impedance is complex and written Zc = -jXc. A real capacitor also includes ESR, ESL, leakage, and other parasitic effects.
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Browse the Capacitors calculator category or review component selection information in the Engineering Reference Center.
This calculator provides ideal capacitive reactance for engineering reference. Verify ESR, ESL, tolerance, leakage, dielectric losses, voltage coefficient, package parasitics, self-resonant frequency, and measured circuit impedance before using the result in production.