A reactance calculator for capacitors, inductors and LC circuits
This reactance calculator gives the capacitive reactance of a capacitor, the inductive reactance of an inductor and the combined impedance of a series or parallel RLC circuit at any frequency. Add both an inductor and a capacitor and it also finds the LC resonant frequency, the Q factor and the bandwidth, and draws how XL, XC and |Z| change across four decades of frequency.
It is meant for students checking AC-circuit homework, hobbyists tuning filters and radios, and engineers sizing a capacitor or choke for mains, audio or RF work. Values accept SI prefixes from pF to F and Hz to GHz, and everything is worked out in your browser.
How to use the capacitive and inductive reactance calculator
- Choose Series RLC or Parallel RLC.
- Enter the frequency and pick its unit (Hz, kHz, MHz or GHz).
- Enter the inductance, capacitance and resistance you have; leave a box empty to leave that part out.
- Read XL, XC, |Z| and the phase angle, plus f₀, Q and bandwidth when both L and C are present.
- Press Set f to resonance to jump to f₀, or Copy results to keep the numbers.
Reactance & Resonance Calculator features
Capacitive and inductive reactance
XC = 1/(2πfC) and XL = 2πfL at any frequency, with automatic kΩ/MΩ prefixes.
Series and parallel RLC
Complex impedance, magnitude and phase angle, labelled as inductive or capacitive.
Resonance, Q and bandwidth
f₀ = 1/(2π√LC), Q factor and −3 dB bandwidth for both circuit types.
Component to resonate
The capacitance (or inductance) that would resonate at the frequency you entered.
Frequency plot
Log-log curves of XL, XC and |Z| with your frequency marked.
Reactance and resonance formulas
An inductor opposes changes in current, so its reactance rises with frequency: X_L = 2πfL. A capacitor passes faster changes more easily, so its reactance falls: X_C = 1/(2πfC). Both are measured in ohms but shift the current by 90° rather than dissipating power.
In a series RLC circuit Z = R + j(X_L − X_C), |Z| = √(R² + (X_L − X_C)²) and the phase angle is φ = arctan((X_L − X_C)/R). The reactances cancel at the resonant frequency f₀ = 1/(2π√(LC)). The quality factor is Q = (1/R)·√(L/C) for series and Q = R·√(C/L) for parallel circuits, and the bandwidth between the half-power points is f₀/Q.
Example: 10 mH, 2.2 µF and 10 Ω in series at 1 kHz. XL = 62.83 Ω, XC = 72.34 Ω, so the net reactance is −9.51 Ω and |Z| = 13.80 Ω at −43.6° (capacitive). Resonance is at 1.073 kHz with Q = 6.74 and a bandwidth of 159 Hz.
Reactance at a glance
| Part | 50 Hz | 60 Hz | 1 kHz | 1 MHz |
|---|---|---|---|---|
| 1 µF capacitor | 3.18 kΩ | 2.65 kΩ | 159 Ω | 0.159 Ω |
| 100 nF capacitor | 31.8 kΩ | 26.5 kΩ | 1.59 kΩ | 1.59 Ω |
| 10 mH inductor | 3.14 Ω | 3.77 Ω | 62.8 Ω | 62.8 kΩ |
| 1 µH inductor | 0.314 mΩ | 0.377 mΩ | 6.28 mΩ | 6.28 Ω |
Practical notes
- Series resonance gives minimum impedance and maximum current; parallel resonance gives maximum impedance, as used in tank circuits and band-stop traps.
- Real inductors have winding resistance (include it in R) and self-resonance; capacitors have ESR and lead inductance that dominate at radio frequencies.
- Voltages across L and C in a series circuit at resonance can be Q times the supply voltage — rate parts accordingly.
Reactance & Resonance Calculator FAQ
How do I calculate capacitive reactance?
XC = 1/(2πfC). A 0.1 µF capacitor at 60 Hz has XC = 1/(2π × 60 × 0.1 × 10⁻⁶) = 26.5 kΩ; at 1 kHz it is 1.59 kΩ.
How do I calculate inductive reactance?
XL = 2πfL. A 10 mH inductor at 1 kHz has XL = 2π × 1000 × 0.01 = 62.8 Ω.
What is an LC reactance calculator used for?
It finds the frequency where an inductor and capacitor resonate, f₀ = 1/(2π√LC), and the reactance of each at a working frequency — the basics of filters, oscillators and radio tuning.
Why is the phase angle negative?
A negative angle means the circuit is capacitive (XC larger than XL), so current leads voltage. Positive means inductive, with current lagging.
What happens at resonance?
XL equals XC. A series circuit then looks like R alone with maximum current; a parallel circuit has maximum impedance.
Does the calculator include losses?
Only through the resistance you enter. Add an inductor’s DC resistance or a capacitor’s ESR to R for a closer estimate.