AC Impedance Calculator (Series RLC)
The impedance, reactance, phase angle and resonant frequency of a series resistor–inductor–capacitor circuit at a given frequency — the AC version of Ohm’s law, where reactance fights the current without burning power.
Set L or C to zero to leave that element out of the circuit.
Ohm’s Law, but for AC
In a DC circuit, resistance alone limits the current. Feed it alternating current and two new effects appear: inductors and capacitors push back on the changing current without dissipating energy. That opposition is reactance, and the total opposition — resistance and reactance combined — is impedance, the quantity that replaces plain resistance in Ohm’s law for AC.
Inductive reactance grows with frequency; capacitive reactance shrinks with it. They act in opposite directions, so the net reactance is their difference, and the impedance is the right-triangle combination of resistance and net reactance.
Phase, Lead and Lag
Reactance also shifts the timing between voltage and current. An inductive circuit makes the current lag the voltage (positive phase angle); a capacitive one makes it lead (negative angle). The cosine of that angle is the power factor — the fraction of the apparent power that does real work. Pure resistance gives a power factor of 1; pure reactance gives 0 and burns nothing.
Resonance
At one special frequency the inductive and capacitive reactances cancel exactly. The net reactance is zero, the impedance drops to just the resistance, and the current peaks. That resonant frequency, f₀ = 1 / (2π√LC), is the basis of radio tuning and filter design.
Frequently Asked Questions
Why does reactance not use power?
Because inductors and capacitors store energy and hand it back each cycle rather than turning it into heat. Only the resistance dissipates real power; reactance just shuttles energy in and out.
What if I only have an R and an L?
Set the capacitance to zero. The tool drops the capacitive term and gives a simple RL impedance and lagging phase. The same works for RC if you zero the inductance.
Is this series or parallel?
Series. The three elements share one current and their voltage drops add as phasors. Parallel RLC combines admittances instead and behaves oppositely at resonance.
Related calculators
- Ohm’s Law Calculator — the DC version with V, I and R.
- Power Factor Correction Calculator — fixing a low power factor from reactance.
- Three-Phase Power Calculator — real, reactive and apparent power.
- Engineering Unit Converter — electrical and other units.