Last updated: 2026-06-16

Capacitive Reactance

Capacitive reactance is the opposition a capacitor presents to alternating current. Like resistance it is measured in ohms, but it depends on frequency: a capacitor passes high-frequency signals readily while strongly opposing low-frequency ones. This calculator takes the signal frequency $f$ and the capacitance $C$ and returns the reactance $X_C$.

Formula

Reactance is inversely proportional to both frequency and capacitance. At DC (zero frequency) the reactance is infinite, so an ideal capacitor blocks direct current once charged.

Worked example

A $1\ \mu\text{F}$ capacitor carries a $1\ \text{kHz}$ signal.

At $2\ \text{kHz}$ the same capacitor has a reactance of about $79.6\ \Omega$ — half as much, because reactance falls as frequency rises.

Reactance at common frequencies

For a $1\ \mu\text{F}$ capacitor:

Why it matters

Capacitive reactance is the basis of filtering, coupling and bypass circuits. A coupling capacitor lets the AC part of a signal through while blocking the DC bias; a bypass capacitor shunts high-frequency noise to ground because its reactance is small at those frequencies. Combined with resistance, capacitive reactance sets the cutoff frequency of RC filters and contributes to the impedance of any AC circuit containing capacitors.

Reactance is not resistance

Although both are measured in ohms, reactance stores and returns energy rather than dissipating it, and it shifts the current 90° ahead of the voltage in an ideal capacitor. To find the total opposition of a circuit that contains resistance and reactance together, combine them as impedance rather than adding them directly.