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Capacitive Reactance Calculator

Find the reactance of a capacitor in an AC circuit.

How it is calculated

The tool computes XC = 1 ÷ (2π × f × C), where f is the frequency in hertz (Hz) and C is the capacitance in farads (F). The answer is in ohms (Ω), rounded to two decimals. Both inputs must be above zero. Capacitance has to be typed in farads, so a 1 µF capacitor is 0.000001 or 1e-6, a 100 nF capacitor is 1e-7 and a 10 pF capacitor is 1e-11. Doubling either the frequency or the capacitance halves the reactance. This is the magnitude only: it ignores resistance, the capacitor's real-world losses (ESR) and the 90 degree phase shift between voltage and current.

Worked example

A 1 µF capacitor on the 60 Hz mains frequency. C = 1e-6 F. Denominator: 2π × 60 × 0.000001 = 0.000377. Reactance: 1 ÷ 0.000377 = 2,652.58 Ω. At 120 Hz, with the same capacitor, the reactance would drop to about half, 1,326 Ω.

Try this example →

Common mistakes

  • Typing the capacitance as 1 when the part is 1 µF. The field is in farads, so 1 means one full farad and gives a tiny reactance; use 0.000001 or 1e-6.
  • Entering the frequency in kHz or MHz. It must be in Hz, so 10 kHz is 10000.
  • Confusing µF, nF and pF by a factor of 1000 when converting. 100 nF is 1e-7 F, not 1e-6.
  • Treating reactance like resistance in power calculations. A capacitor does not dissipate power in the ideal case; the current leads the voltage by 90 degrees.
  • Expecting a value at 0 Hz. DC has infinite reactance, and the tool gives no result for zero frequency.

About this calculator

Capacitive reactance measures the opposition a capacitor presents to alternating current: Xc = 1 / (2πfC), where f is frequency and C is capacitance. Unlike a resistor, a capacitor's opposition to current isn't constant — it falls as frequency rises (a capacitor passes high frequencies more easily) and rises toward infinity as frequency approaches zero (a capacitor blocks DC entirely).

Electrical and electronics engineers use this relationship to design filters — a capacitor in the right configuration builds a low-pass or high-pass filter purely from this frequency-dependent behavior — as well as for impedance matching and power-factor correction in AC circuits. Students studying AC circuit analysis and hobbyist circuit designers use this calculator to check reactance at a chosen frequency before building or simulating a circuit.

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