Find a series RLC circuit's reactance, total impedance, and phase angle at any frequency.

How to Use the RLC Impedance Calculator

Enter resistance, inductance (in millihenries), capacitance (in microfarads), and the signal frequency. The calculator converts inductance and capacitance to base units internally and returns inductive reactance, capacitive reactance, total impedance, and the phase angle between voltage and current.

A series circuit with a resistor, inductor, and capacitor doesn't resist current the same simple way a plain resistor does — inductors and capacitors both push back against changing current, just in opposite directions, and how much they push back depends on frequency rather than staying fixed. Four formulas cover the full picture:

XL = 2πfL
XC = 1 ÷ (2πfC)
Z = √(R² + (XL − XC)²)
θ = atan((XL − XC) ÷ R)

XL, inductive reactance, grows with frequency — an inductor fights harder against fast-changing current. XC, capacitive reactance, does the opposite, shrinking as frequency rises, since a capacitor charges and discharges more freely when it doesn't have time to fully fill up between cycles. Total impedance Z combines resistance with the net difference between the two reactances, and the phase angle θ tells you whether current leads or lags voltage in the circuit as a whole.

Resonance: When XL Equals XC

Set frequency so that inductive and capacitive reactance exactly cancel and the reactive term in the impedance formula drops to zero — impedance falls to just R, its lowest possible value for that circuit, and the phase angle sits at exactly 0°. That frequency is the circuit's resonant frequency, and it's the whole operating principle behind radio tuning circuits, filters, and oscillators: pick L and C to resonate at the frequency you want to pass or reject, and everything else follows from there.

A zero-resistance circuit is the one edge case worth a special note. With R at exactly zero, the phase angle would otherwise require dividing by zero — this calculator handles that directly, reporting ±90° whenever the reactances don't cancel (current fully leading or lagging voltage) and 0° in the special case where they do, rather than returning an error for a configuration that's physically valid, if idealized.