Reactance Calculator (Capacitive & Inductive)

Reactance

Reactance is the opposition a capacitor or inductor presents to alternating current, measured in ohms but varying with frequency. Capacitive reactancefalls as frequency rises; inductive reactance rises.

Capacitive: X_c = 1 / (2π·f·C)

Inductive: X_l = 2π·f·L

In a series LC circuit the net reactance is X_l − X_c: positive means the circuit looks inductive, negative means capacitive, and zero is resonance, where they cancel at f₀ = 1 / (2π·√(L·C)).

How it works

A capacitor opposes a change in voltage, so the faster the signal swings, the more current it passes and the lower its reactance — which is why X_c is inversely proportional to frequency. An inductor opposes a change in current, so faster swings meet more opposition and X_l rises with frequency. That opposite behaviour is the whole basis of filters: a capacitor to ground passes highs and blocks lows, while a series inductor does the reverse. At one specific frequency the two reactances are equal and cancel, and the LC pair resonates.

Two worked examples

A 100 nF capacitor at 1 kHz. X_c = 1 / (2π · 1000 · 100×10⁻⁹) ≈ 1592 Ω. Raise the frequency tenfold to 10 kHz and the reactance drops tenfold to about 159 Ω — the same capacitor is a very different "resistor" depending on frequency.

A 100 µH inductor at 1 kHz. X_l = 2π · 1000 · 100×10⁻⁶ ≈ 0.63 Ω, rising to about 6.3 Ω at 10 kHz. Enter both this inductor and the capacitor above and the calculator also reports the resonance point where they meet.

Common mistakes

Reactance is not resistance: it stores and returns energy rather than dissipating it as heat, and it shifts the current 90° out of phase with the voltage. It's also meaningless without a frequency — the same part has a different reactance at every frequency, so always state the frequency you're working at. To combine reactance with real resistance into a single figure you need impedance (the vector sum), not a plain addition.

Where it's used

Find the tuning point with theLC resonant frequency calculator, or combine parts first with thecapacitor andinductor series & parallelcalculators.

Ideal components only

These formulas describe a pure capacitance or a pure inductance. Real parts carry equivalent series resistance and their own parasitic reactance, and every component has a self-resonant frequency above which it stops behaving like the thing on the schematic — a capacitor becomes inductive, an inductor becomes capacitive. For decoupling and filtering near those frequencies, the impedance curve on the datasheet is the authority rather than the calculated reactance.