Impedance Calculator
Calculate AC impedance, reactance, phase angle, resonance, power factor, current, and circuit behavior for RC, RL, LC, and RLC circuits.
Calculator is for informational purposes only. Terms and Conditions
Choose what to solve for
Select the circuit type, connection, and target result.
Enter the known values
Fill in the visible fields. The calculator updates automatically.
Circuit and phasor visual
The visual updates with the selected circuit and calculated impedance angle.
Solution
Live result, quick checks, warnings, and calculation steps.
Quick checks
- Complex impedance—
- Phase angle—
- Net reactance—
- Inductive reactance—
- Capacitive reactance—
- Circuit behavior—
Source, standards, and assumptions
Educational AC circuit model
- Source/standard: Standard engineering formula or educational calculation method. No single governing code standard is required for this simplified calculation.
- Calculation basis: Ideal sinusoidal steady-state AC circuit analysis using complex impedance.
- Series circuits add impedance directly. Parallel circuits add branch admittance and invert the total.
- This is a lumped-element AC circuit calculator, not a PCB trace, microstrip, stripline, differential pair, antenna, or transmission-line impedance calculator.
Show solution steps See known values, equations, substitutions, and interpretation
- Enter values to see the full solution steps and checks.
What Is Impedance?
Impedance is the total opposition an AC circuit presents to current. It includes ordinary resistance plus frequency-dependent reactance from inductors and capacitors. Resistance is the real part of impedance, while reactance is the imaginary part. The result is commonly written as a complex value, such as Z = R + jX.
Use this impedance calculator when you want to calculate AC circuit impedance, RLC impedance, reactance, phase angle, power factor, resonant frequency, or the capacitance or inductance needed for resonance. The calculator above is designed for ideal lumped-element circuits, not PCB trace impedance or transmission-line impedance.
Direct answer
Impedance is calculated by combining resistance and reactance as a complex number. For a series RLC circuit, the common formula is Z = R + j(XL − XC), where XL = 2πfL and XC = 1/(2πfC).
Impedance Formula
The most useful impedance formulas depend on whether the circuit is series or parallel. In a series circuit, component impedances are added directly. In a parallel circuit, the easiest method is to add admittances first, then invert the result.
Series RLC Impedance
Use this equation when the resistor, inductor, and capacitor are connected in one path and the same current flows through each component.
Impedance Magnitude
The magnitude tells you the size of the impedance in ohms. It is the value most users expect when they ask for “total impedance.”
Parallel Impedance
In parallel AC circuits, admittance is usually cleaner than trying to combine impedance values directly.
Reactance and Resonance
Inductive reactance increases with frequency. Capacitive reactance decreases with frequency. Resonance occurs when the two are equal in magnitude.
What the Impedance Variables Mean
Most wrong impedance calculations come from mixing units, using frequency incorrectly, or forgetting that capacitor reactance is negative in the complex impedance model. The table below summarizes the variables used by the calculator.
| Symbol | Meaning | What to Enter or Review |
|---|---|---|
| Z | Impedance | Total AC opposition to current, expressed as a complex value or magnitude in ohms |
| R | Resistance | The real part of impedance; dissipates energy as heat |
| X | Reactance | The imaginary part of impedance from inductors and capacitors |
| XL | Inductive reactance | Positive reactance from an inductor; increases as frequency increases |
| XC | Capacitive reactance | Capacitive opposition; decreases as frequency increases and appears negative in impedance |
| f | Frequency | The AC signal frequency in Hz, kHz, MHz, or GHz |
| L | Inductance | Entered in pH, nH, µH, mH, or H depending on the scale of the circuit |
| C | Capacitance | Entered in pF, nF, µF, mF, or F depending on the scale of the circuit |
| θ | Phase angle | Shows whether the circuit behaves more inductively, capacitively, or resistively |
How to Use the Impedance Calculator
The calculator above is built for the way people actually search for this topic: some users need a quick impedance magnitude, while others need the complex result, phase angle, resonant frequency, or a missing L or C value for resonance.
Select the circuit type
Choose R, C, L, RC, RL, LC, or RLC. This controls which inputs are visible and which formulas are used.
Choose series or parallel
For a series circuit, impedance values add directly. For a parallel circuit, the calculator adds admittance and then converts back to impedance.
Enter R, L, C, and frequency
Use the unit dropdowns carefully. Electronics problems commonly mix ohms, microhenries, nanofarads, and kilohertz.
Review the complex result
The magnitude tells you the size of impedance, but the complex form and phase angle tell you whether the circuit is inductive, capacitive, or mostly resistive.
Use the optional voltage input for current and power
If you enter RMS voltage, the calculator estimates RMS current, apparent power, real power, reactive power, and power factor.
Important calculator limitation
This calculator assumes an ideal lumped-element circuit. It does not calculate PCB trace impedance, microstrip impedance, stripline impedance, antenna impedance, or transmission-line behavior.
Series vs. Parallel Impedance
Series and parallel AC circuits can contain the same components but produce very different impedance results. This is why the calculator includes a connection selector instead of treating every RLC circuit the same way.
| Circuit Type | Best Calculation Method | What Happens Near Resonance |
|---|---|---|
| Series RC, RL, LC, or RLC | Add component impedances directly | In a series RLC circuit, impedance can dip near resonance because inductive and capacitive reactance cancel |
| Parallel RC, RL, LC, or RLC | Add admittances, then invert | In an ideal parallel LC/RLC circuit, impedance can peak near resonance because branch susceptances cancel |
Engineering interpretation
In a series circuit, a low impedance can allow high current. In a parallel circuit, a high impedance can occur near resonance even though large currents may circulate inside the inductor and capacitor branches.
How Reactance Changes With Frequency
Impedance is important because inductors and capacitors do not behave like simple fixed resistors. Their reactance changes with frequency, which is why the same circuit can behave differently at 60 Hz, 1 kHz, 1 MHz, or 1 GHz.
Inductor
XL = 2πfL. As frequency rises, inductive reactance rises.
Capacitor
XC = 1/(2πfC). As frequency rises, capacitive reactance falls.
RLC Circuit
At one frequency, the inductor and capacitor can cancel each other’s reactance.
This frequency dependence is why impedance matters in filters, audio crossovers, resonant tanks, power factor problems, RF circuits, and AC circuit homework. Khan Academy summarizes the same relationship: inductor impedance is directly proportional to frequency, while capacitor impedance is inversely proportional to frequency. Review impedance versus frequency.
How to Read the Phasor Diagram
A phasor diagram is one of the clearest ways to understand impedance. The horizontal axis represents the real resistance part of impedance. The vertical axis represents reactance. The diagonal vector represents the total impedance.
Positive angle
A positive phase angle usually means the circuit is inductive. Current lags voltage.
Negative angle
A negative phase angle usually means the circuit is capacitive. Current leads voltage.
Near zero angle
A phase angle near zero means the circuit is mostly resistive or near resonance.
Magnitude
The length of the impedance vector is the impedance magnitude, written as |Z|.
The calculator’s phasor visual updates as the result changes so users can connect the numeric phase angle to the physical circuit behavior.
Step-by-Step Worked Example
The following example shows how a typical series RLC impedance calculation works. This mirrors what the calculator does automatically.
Calculate Reactance
Combine the Impedance
Result
Impedance magnitude: approximately 1594.63 Ω. The circuit is strongly capacitive at 1 kHz because capacitive reactance is much larger than inductive reactance.
How to Interpret It
Even though the resistor is 100 Ω, the capacitor dominates at this frequency. Increasing the frequency would reduce capacitive reactance and move the circuit closer to resonance.
Resonance in RLC Circuits
Resonance occurs when inductive reactance and capacitive reactance are equal in magnitude. At that frequency, the reactive effects cancel in the ideal model.
Resonant Frequency
The calculator can solve directly for resonant frequency when inductance and capacitance are known.
Capacitance Needed for Resonance
Use this when you know the target frequency and inductor value.
Inductance Needed for Resonance
Use this when you know the target frequency and capacitor value.
Series and parallel resonance are not the same
In a series RLC circuit, impedance reaches a minimum near resonance. In an ideal parallel resonant circuit, impedance can reach a maximum. LibreTexts describes series RLC resonance as the condition where inductive reactance equals capacitive reactance and impedance is minimized. Review AC resonance.
Power Factor and Phase Angle
If you enter RMS voltage into the calculator, the tool can estimate current and AC power values. This is useful when impedance is being used to understand current draw, apparent power, real power, or reactive power.
| Output | Meaning | Why It Matters |
|---|---|---|
| Phase angle | Angle between voltage and current | Shows whether current leads or lags voltage |
| Power factor | cosine of the phase angle magnitude | Indicates how effectively apparent power becomes real power |
| Apparent power | Voltage multiplied by current | Measured in volt-amperes, or VA |
| Real power | Power dissipated by the resistive part | Measured in watts, or W |
| Reactive power | Power exchanged with fields in L and C | Measured in volt-ampere reactive, or VAR |
A positive phase angle usually means an inductive circuit. A negative phase angle usually means a capacitive circuit. A phase angle near zero means the circuit is mostly resistive.
Practical Limits of an Ideal Impedance Calculation
A calculator can return a mathematically correct impedance while still missing important real-world behavior. This is especially true at high frequency, near resonance, or when using real capacitors and inductors.
Capacitor ESR
Real capacitors have equivalent series resistance, leakage, dielectric loss, and voltage-dependent behavior.
Inductor winding resistance
Real inductors have copper resistance, core losses, saturation limits, and self-resonant frequency.
PCB layout effects
At high frequencies, traces, vias, ground return paths, and component placement can dominate the result.
Transmission-line behavior
Microstrip, stripline, coax, antennas, and differential pairs require transmission-line impedance methods.
Engineering judgment
Use the calculator for ideal AC circuit analysis, homework, early design checks, and conceptual comparison. For final RF, PCB, power electronics, or high-current design, verify with component datasheets, measurement, simulation, and layout-aware analysis.
Common Impedance Calculator Mistakes
These are the most common reasons users get an impedance result that looks correct but does not match the physical circuit.
Common Don’ts
- Use capacitance in µF while the formula assumes farads
- Forget that capacitive reactance is negative in complex impedance
- Treat series and parallel RLC circuits as if they use the same formula
- Assume resonance always means low impedance
- Ignore ESR, winding resistance, and layout effects at high frequency
- Use this calculator for PCB trace impedance or differential pair impedance
Better Checks
- Use the calculator’s unit dropdowns instead of manually converting everything
- Review both impedance magnitude and complex impedance
- Check the phase angle to identify inductive or capacitive behavior
- Compare the entered frequency to the resonant frequency
- Add real resistance when modeling real inductors or lossy circuits
- Use PCB or RF-specific tools when geometry and transmission lines matter
Frequently Asked Questions
What does an impedance calculator do?
An impedance calculator finds the total AC opposition of a circuit. A good calculator should show impedance magnitude, complex impedance, phase angle, reactance, resonance, and circuit behavior.
What is the formula for impedance in a series RLC circuit?
For a series RLC circuit, impedance is commonly written as Z = R + j(XL − XC). The magnitude is |Z| = √[R² + (XL − XC)²].
How do you calculate capacitive reactance?
Capacitive reactance is calculated as XC = 1/(2πfC). It decreases as frequency or capacitance increases.
How do you calculate inductive reactance?
Inductive reactance is calculated as XL = 2πfL. It increases as frequency or inductance increases.
What does a negative impedance angle mean?
A negative impedance phase angle usually means the circuit is capacitive overall. In that case, current leads voltage.
What does a positive impedance angle mean?
A positive impedance phase angle usually means the circuit is inductive overall. In that case, current lags voltage.
What happens to impedance at resonance?
In an ideal series RLC circuit, impedance is minimized near resonance. In an ideal parallel LC or RLC circuit, impedance can become very high near resonance.
Is impedance the same as resistance?
No. Resistance is only the real part of impedance. Impedance includes both resistance and reactance, so it can have magnitude and phase.
Can this calculator be used for PCB trace impedance?
No. This calculator is for lumped-element AC circuits. PCB trace impedance, microstrip, stripline, and differential pair impedance require geometry-based transmission-line calculations.
Why is impedance frequency dependent?
Impedance is frequency dependent when inductors or capacitors are present. Inductor reactance increases with frequency, while capacitor reactance decreases with frequency.