⚡ Reactance Calculator

Capacitors and inductors oppose the flow of alternating current not by resistance but by reactance — a frequency-dependent opposition measured in ohms. A capacitor's reactance is Xₘ = 1/(2πfC): it passes high frequencies easily and blocks low ones. An inductor's reactance is Xₙ = 2πfL: it does the opposite. Together they are the frequency-selective building blocks of every filter, crossover and tuned circuit.

Reactance is why a tweeter and a woofer in the same speaker box can each get the right part of the audio spectrum, why a capacitor blocks DC in a coupling stage, and why an inductor smooths a power supply. Understanding the two curves — one rising, one falling — is the first step to designing any frequency-selective circuit.

The calculator takes frequency (with Hz/kHz/MHz/GHz selection) and either capacitance or inductance, converts to base units, and applies Xₘ = 1/(2πfC) or Xₙ = 2πfL. If both L and C are entered it also computes the resonant frequency where the two reactances are equal — the crossover of the two curves — which is the same f₀ = 1/(2π√(LC)) used by our LC resonance calculator.

The derived reactance in kΩ and MΩ helps you pick the right component size quickly, and the LC resonance cross-check is handy when pairing a capacitor with an inductor for a tuned circuit. For impedance of the whole circuit (resistance plus reactance) combine the result with Ohm's law and our other electronics calculators.

1. Choose the component type: capacitor or inductor.
2. Enter the component value and its unit (pF–F or nH–H).
3. Enter the frequency and its unit (Hz, kHz, MHz or GHz).
4. Click Calculate to see the reactance in ohms, kΩ and MΩ.
5. If both L and C are entered, read the resonance cross-check too.

What is the difference between resistance and reactance?

Resistance opposes current in the same way at all frequencies and dissipates energy as heat. Reactance opposes current by storing and returning energy (in an electric or magnetic field) and depends on frequency. Together they form impedance, Z = R + jX.

Why does capacitive reactance fall as frequency rises?

A capacitor passes charge by charging and discharging. At higher frequency the cycles are shorter, so more charge flows per second for the same voltage — effectively less opposition. At DC (0 Hz) Xc is infinite, which is why a capacitor blocks DC.

At what frequency do XC and XL become equal?

The frequency where the reactances match is the resonance f₀ = 1/(2π√(LC)). The calculator shows it when both values are entered. Below f₀ the circuit behaves capacitively; above it, inductively.

Is reactance the same as impedance?

No. Impedance is the vector sum of resistance and reactance, Z = √(R² + X²) with a phase angle. Reactance is only the imaginary part. For a pure capacitor or inductor (R = 0) the two are numerically equal, but real components always have some resistance.

How do I choose a capacitor value for a coupling circuit?

Pick Xc well below the impedance of the following stage at the lowest frequency you must pass — typically 10× smaller or less. For audio coupling, Xc under about 160 Ω at 100 Hz needs C > 10 µF, which is why audio circuits use microfarad-class capacitors.

Why does my measured reactance differ from the calculation at high frequencies?

Real components have parasitic elements: capacitor ESL and ESR, inductor inter-winding capacitance and DCR. Above the self-resonant frequency the parts behave differently entirely. Calculations hold for frequencies comfortably below SRF.

Remember the phase behaviour: current leads voltage by 90° in a capacitor and lags by 90° in an inductor — reactance alone never tells the whole story, but the magnitude is what filters are cut at. When reading a capacitor at high frequency, check for self-resonance: above its SRF the part stops behaving as a capacitor and looks inductive. For an inductor, the DC resistance (DCR) is usually small but matters in high-current filters. Reactance numbers below about 10 Ω or above 1 MΩ are impractical for real circuits because stray resistance and capacitance start to dominate — design in the sensible range. In an audio crossover, doubling the frequency doubles XL but halves XC, which is why speaker systems use steep-slope networks with both parts per driver.