Capacitive Reactance Calculator
Capacitive reactance Xc = 1 ÷ (2πfC) for any capacitance and frequency — with the current at a given voltage, phase relationship and a frequency sweep table.
How it works
Xc = 1 ÷ (2π f C); I = V ÷ Xc; Xc halves each time the frequency doubles
A capacitor opposes alternating current not by dissipating energy but by storing and returning it every half cycle, and the size of that opposition — its reactance — shrinks as frequency rises, because a faster-changing voltage moves more charge per second. At mains frequency a 10 µF capacitor presents about 318 Ω; at 10 kHz the same part is 1.6 Ω, which is why capacitors pass audio and RF while blocking DC (infinite reactance at 0 Hz). The current leads the voltage by a quarter cycle, so the power is purely reactive. Reactance combines with resistance as an impedance vector, not a simple sum, and with inductive reactance it cancels at resonance.
Worked example
10 µF at 50 Hz: Xc = 1 / (2π × 50 × 10⁻⁵) = 318 Ω; across 230 V it passes 0.72 A of purely reactive current.
Frequently asked questions
Does a capacitor have resistance too?
A small equivalent series resistance (ESR) — milliohms for film types, more for electrolytics — which dissipates real power and matters at high ripple currents.
Why is reactance given as −j?
Complex notation: capacitive reactance is negative imaginary, inductive positive imaginary; they add algebraically so an LC pair can cancel.
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This calculator is for educational purposes. Double-check important results before acting on them.