Last updated: 2026-06-16

Inductive Reactance

Inductive reactance is the opposition a coil presents to alternating current. Like resistance it is measured in ohms, but it depends on frequency: an inductor passes low-frequency signals readily while strongly opposing high-frequency ones. This calculator takes the signal frequency $f$ and the inductance $L$ and returns the reactance $X_L$.

Formula

Reactance is directly proportional to both frequency and inductance. At DC (zero frequency) the reactance is zero, so an ideal inductor behaves like a plain wire for direct current once the current has settled. This is the opposite of a capacitor, whose reactance falls as frequency rises.

Worked example

A $10\ \text{mH}$ inductor carries a $1\ \text{kHz}$ signal.

At $2\ \text{kHz}$ the same inductor has a reactance of about $125.7\ \Omega$ — twice as much, because reactance rises as frequency rises.

Reactance at common frequencies

For a $10\ \text{mH}$ inductor:

Why it matters

Inductive reactance is the basis of chokes, filters and impedance matching. A choke uses its rising reactance to block high-frequency noise while letting DC and low frequencies pass. Combined with capacitance, inductive reactance sets the resonant frequency of LC circuits, and together with resistance it contributes to the impedance of any AC circuit containing coils.

Reactance is not resistance

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