Capacitive Reactance

How to use this calculator

Enter the AC signal frequency and the capacitance, and the capacitive reactance X_C = 1 / (2π f C) is computed instantly in the most readable unit. Reactance is the AC "opposition" a capacitor presents — it falls as both frequency and capacitance rise, so a large capacitor or a high-frequency signal sees a low reactance, while a tiny capacitor at low frequency looks almost like an open circuit.

What the value means in a circuit

Unlike a resistor, a capacitor's reactance depends on frequency, which is exactly what makes capacitors useful in AC work: coupling (passing the signal while blocking DC), filtering (choosing which frequencies get through), and timing with resistors (the RC corner at f = 1/2πRC). Reactance also has a phase effect — current leads voltage by 90° — so X_C is used to build an impedance (Z = −jX_C) when analysing filters, power factor and the response of AC circuits.