Capacitive Reactance Calculator

Calculate capacitive reactance (Xc) from signal frequency and capacitance using the standard AC circuit formula Xc = 1 / (2πfC).

Quick Facts

Formula
Xc = 1 / (2πfC)
Reactance is inversely proportional to both frequency and capacitance.
At the extremes
Blocks DC, passes high frequencies
As f approaches 0, Xc approaches infinity; as f grows large, Xc approaches 0.

Your Results

Calculated
Capacitive reactance (Xc)
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Opposition to AC current
Angular frequency (ω)
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ω = 2πf, in rad/s
Capacitance used
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Converted to farads
Signal period (T)
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T = 1/f, one full cycle

Ready

Enter frequency and capacitance, then press Calculate.

Understanding Capacitive Reactance

Capacitive reactance (Xc) is the opposition a capacitor presents to alternating current, measured in ohms just like resistance. Unlike a resistor, a capacitor's opposition to current changes with the frequency of the signal: it blocks low-frequency and DC current while letting high-frequency current pass with little opposition. This calculator applies the standard AC circuit formula to any frequency and capacitance you enter.

The formula

Capacitive reactance is calculated as:

Xc = 1 / (2πfC)

where f is the signal frequency in hertz (Hz) and C is the capacitance in farads (F). The term 2πf is the angular frequency ω, so the formula is often written Xc = 1/(ωC). Because f and C are both in the denominator, Xc shrinks as either frequency or capacitance increases, and grows without bound as either approaches zero.

Working with units

  • Frequency is entered in hertz (Hz) — cycles per second.
  • Capacitance is entered in your chosen unit and converted to farads internally: 1 µF = 10-6 F, 1 nF = 10-9 F, 1 pF = 10-12 F.
  • The result is reported in ohms (Ω), automatically scaled to kilo-ohms (kΩ) or mega-ohms (MΩ) when the value is large, or milliohms (mΩ) when it is very small.

Knowing the limits

This formula assumes an ideal capacitor operating in sinusoidal steady-state AC — no equivalent series resistance (ESR), no lead inductance, and a single fixed frequency. Real capacitors deviate from ideal behavior at very high frequencies, where parasitic inductance can make them behave inductively instead of capacitively. For a full picture of an AC branch, combine Xc with any resistance R using the impedance formula Z = √(R² + Xc²).

Frequently Asked Questions

What is capacitive reactance?
Capacitive reactance (Xc) is the opposition a capacitor presents to alternating current, measured in ohms. It is calculated as Xc = 1 / (2πfC), where f is frequency in hertz and C is capacitance in farads. Unlike resistance, reactance depends on frequency and an ideal capacitor does not dissipate energy as heat.
How does frequency affect capacitive reactance?
Capacitive reactance is inversely proportional to frequency: as frequency rises, Xc falls. At very high frequencies a capacitor behaves almost like a short circuit (near-zero reactance), while near DC (very low frequency) its reactance approaches infinity, which is why capacitors block DC current.
What units does this calculator use?
Enter frequency in hertz (Hz) and capacitance in picofarads, nanofarads, microfarads, or farads. The calculator converts your capacitance to farads internally before applying Xc = 1 / (2πfC), then reports the reactance in ohms, automatically scaling to kilo-ohms or mega-ohms for large values.
How is capacitive reactance different from resistance?
Both are measured in ohms, but resistance dissipates energy as heat regardless of frequency, while reactance stores and releases energy in the capacitor's electric field and changes with frequency. In an AC circuit, resistance and reactance combine as impedance Z = √(R² + Xc²) when in series.

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