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Capacitor Impedance at Frequency

Capacitive reactance from frequency and capacitance

Result

15.92 Ω

How it is calculated

The tool calculates capacitive reactance Xc = 1 / (2π × f × C). f is the frequency in hertz (Hz). C is the capacitance, which this tool takes in microfarads (µF); it divides by 1,000,000 to get farads before applying the formula. Xc is returned in ohms (Ω), rounded to two decimals.

Both frequency and capacitance must be greater than zero. At 0 Hz (DC) the reactance is theoretically infinite, so the tool gives no result. The value is the magnitude of the opposition only; for a capacitor the current leads the voltage by 90°, and an ideal capacitor is assumed with no series resistance or inductance.

Worked example

A 10 µF capacitor at 1,000 Hz. Convert to farads: 10 µF = 0.00001 F. Then 2π × 1,000 × 0.00001 ≈ 0.062832. The reactance is 1 / 0.062832 ≈ 15.92. The tool shows 15.92 Ω. At 50 Hz the same capacitor presents 318.31 Ω, twenty times higher, because the frequency is twenty times lower.

Try this example →

Common mistakes

  • Typing the capacitance in farads. This tool expects microfarads, so 0.00001 would be read as a tiny 0.00001 µF capacitor and give a huge answer.
  • Entering the frequency in kHz or MHz without converting. The field is in hertz, so 1 kHz is 1000.
  • Treating the value like a resistance in a DC calculation. Xc applies to AC only and shrinks as the frequency rises.
  • Adding capacitive reactance directly to a resistance. In series they combine as a vector sum (√(R² + Xc²)), not by simple addition.
  • Ignoring real-world parts. Actual capacitors have series resistance and inductance, so the measured impedance differs at high frequency.

Sources

About this calculator

This calculator finds a capacitor's capacitive reactance, Xc = 1 / (2πfC), where f is frequency in hertz and C is capacitance in farads. You enter the capacitance in microfarads (µF), the unit printed on most capacitors, and the tool converts it to farads. Reactance is a capacitor's frequency-dependent opposition to AC current — unlike a resistor, it isn't constant: reactance drops as frequency rises, which is exactly why capacitors block DC (f = 0, infinite reactance) but pass high frequencies easily.

Audio engineers use it to design crossover networks that route treble to tweeters and bass to woofers, RF and analog circuit designers use it to size decoupling and filter capacitors for a target cutoff frequency, and electronics students use it to verify hand calculations on AC circuit homework.

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Comments (1)

  • Editorial team

    This gives the reactance at one frequency, but it also helps you spot a filter corner. A capacitor in series with a load acts as a high-pass filter around the frequency where its reactance equals the load resistance. For 10 µF and an 8 Ω speaker, entering 1989 Hz gives 8.00 Ω, so the corner is near 2 kHz. Real capacitors add some resistance. Which circuit are you sizing, and with what values?

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