AC Impedance Calculator (RLC)
Enter values for resistance, inductance, capacitance, and frequency to calculate complex impedance, magnitude, phase angle, and power factor.
What Is Impedance?
Impedance is the total opposition a circuit presents to alternating current (AC). It combines ordinary resistance with frequency-dependent reactance from inductors and capacitors. Unlike pure resistance, impedance includes both magnitude and phase, which is why it is written as a complex quantity:
Z = R + jX
Where R is resistance (real part), X is reactance (imaginary part), and j represents the square root of negative one in electrical engineering notation.
Core Formulas Used by the Calculator
Component Reactance
- Inductive reactance: XL = 2πfL
- Capacitive reactance: XC = 1 / (2πfC)
As frequency increases, inductive reactance rises while capacitive reactance drops. This is why AC circuit behavior changes with frequency.
Series RLC Impedance
For a series RLC network, reactances combine directly:
X = XL − XC
|Z| = √(R² + X²)
The phase angle is θ = tan-1(X/R). Positive angle means inductive (current lags). Negative angle means capacitive (current leads).
Parallel RLC Impedance
For a parallel network, admittance is easier to compute first:
- Conductance: G = 1/R
- Susceptance: B = 1/XC − 1/XL
Then convert admittance back to impedance to get the real and imaginary parts of Z, its magnitude, and phase angle.
How to Use This Impedance Calculator
- Select Series RLC or Parallel RLC.
- Enter resistance in ohms, inductance in millihenries, capacitance in microfarads, and frequency in hertz.
- Click Calculate Impedance.
- Review complex impedance, magnitude, phase, power factor, and resonance frequency (if both L and C are present).
Why Impedance Matters in Real Designs
Impedance determines current draw, voltage drop, and power transfer in AC circuits. You see it everywhere: speaker systems, filters, power supplies, RF matching networks, and motor drives. If impedance is mismatched, circuits run inefficiently, produce extra heat, and can distort signals.
Common Mistakes to Avoid
- Mixing units (mH vs H, µF vs F).
- Forgetting that reactance depends on frequency.
- Treating AC impedance like DC resistance only.
- Ignoring phase angle when calculating power factor or true power.
Quick FAQ
What happens at resonance?
At resonance in a series RLC circuit, XL equals XC, so reactive effects cancel and impedance is mostly resistive. Resonant frequency is fr = 1 / (2π√LC).
Can impedance be purely real?
Yes. That occurs when net reactance is zero. In that case the phase angle is near 0°, and power factor approaches 1.
Is lower impedance always better?
Not necessarily. The “right” impedance depends on the system objective: maximum power transfer, signal integrity, filtering behavior, efficiency, or safe current limits.