Impedance Calculator
Complex impedance
Choose series or parallel, enter frequency and any of R, L, C; magnitude and phase appear live.
Impedance
How to use this calculator
Impedance generalizes resistance to AC: a resistor has Z = R, a capacitor has Z = 1/(jωC) (magnitude 1/ωC, phase −90°), and an inductor has Z = jωL (magnitude ωL, phase +90°), where ω = 2πf. For series combinations the impedances add; for parallel combinations the admittances add (Z = 1/Σ(1/Zi)). Choose the configuration, enter the frequency and any of R, L, C, and the calculator shows the total impedance magnitude, phase, and real/reactive parts.
What it means in a real circuit
For a series 100 Ω resistor and 10 nF capacitor at 1 kHz: the capacitor's reactance is 1/(2π×1000×10 nF) ≈ 15.9 kΩ, so |Z| ≈ 15.9 kΩ with a phase near −90° (mostly capacitive). At resonance in an RLC circuit the inductive and capacitive reactances cancel and the impedance is purely resistive. Impedance calculations are essential for matching, filtering, and understanding how a circuit's behavior changes with frequency. Use this to find the input/output impedance of a stage, to design a matching network, or to check that a load won't overload a source.
Definitions
Impedance (Z): the complex opposition to AC, Z = R + jX. Reactance (X): the frequency-dependent part — capacitive (Xc = 1/ωC, negative) or inductive (Xl = ωL, positive). Magnitude: |Z| = √(R² + X²). Phase: φ = atan2(X, R) — the angle by which voltage leads (inductive) or lags (capacitive) current. Admittance (Y): 1/Z, used for parallel combinations.
Design tips
For matching, aim for the load impedance to equal the source's (conjugate match for maximum power). For filtering, combine R, L, C to shape the frequency response. When in doubt, compute |Z| at the frequencies of interest and check that it's in the right range. For parallel RLC, the impedance peaks at resonance (high Q); for series RLC, it's minimal at resonance (equal to R).
Common mistakes
Mixing up the series and parallel combination rules (add Z for series, add 1/Z for parallel). Forgetting that capacitive reactance is negative and inductive is positive, so they can cancel. Using DC resistance where AC impedance is needed. Ignoring that a capacitor's impedance is infinite at DC and an inductor's is zero at DC. Confusing impedance (Ω) with admittance (siemens).
Component-selection advice
Choose R, L, C from standard values that get you close to the target |Z| and phase, then iterate. For high-frequency work, use components with low parasitics (film caps, air-core inductors). Verify the real impedance with an LCR meter, especially above a few MHz where parasitics dominate.
Limitations of this calculation
The model uses ideal R, L, C with no parasitic resistance, capacitance, or inductance. At high frequency, real components deviate significantly (a capacitor looks inductive above its self-resonant frequency; an inductor looks capacitive). The calculation assumes a single frequency — for broadband signals, you'd need to evaluate |Z| over a range. It also doesn't model mutual inductance or nonlinear (voltage-dependent) behavior.
Example use cases
Designing a matching network for an RF amplifier; finding the input impedance of a filter stage; checking that a speaker load matches an amplifier; sizing a snubber or damping resistor; understanding how a circuit's response changes with frequency.
Frequently asked questions
Impedance (Z) is the complex opposition a circuit presents to AC, measured in ohms. It has a real part (resistance) and an imaginary part (reactance), and it depends on frequency.
A capacitor's reactance is Xc = 1/(2πfC). At low f, Xc is large (blocks current); at high f, Xc is small (passes current). At DC (f = 0) it's infinite — an open circuit.
An inductor's reactance is Xl = 2πfL. At low f, Xl is small (passes current); at high f, Xl is large (blocks current). At DC (f = 0) it's zero — a short circuit.
Series: add the impedances (Z = Z1 + Z2). Parallel: add the admittances (1/Z = 1/Z1 + 1/Z2), then invert. The calculator handles both.
It's the angle by which voltage leads (positive, inductive) or lags (negative, capacitive) the current. 0° means purely resistive; +90° purely inductive; −90° purely capacitive.
Ideal — they assume no parasitic resistance, capacitance, or inductance. At high frequency real components deviate; verify with an LCR meter for critical designs.