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Impedance of a Resistor

In AC circuits, a resistor behaves just like it does in DC: its impedance is simply its resistance value, a plain real number. There is no phase shift between voltage and current. So a 100 ohm resistor has an impedance of 100 ohms at any frequency. This makes resistors the simplest components to analyse in AC.

Written by the Vorzep Team

AC Circuits Impedance

Resistor Impedance Calculator

Ideal resistive impedance, voltage/current, power, tolerance and temperature analysis

AC / DC
ZR = R + j0
|ZR| = R  ·  ∠ZR = 0°
Mode
Solve For
Ideal resistance R
V
Optional in impedance mode
Optional in impedance mode
Quick Presets
Common Resistor Reference
Resistance12 V Current12 V Power24 V Current24 V Power
Formula Reference
Resistor Impedance
ZR = R + j0
Magnitude
|ZR| = R
Phase
∠ZR = 0°
Ohm's Law
I = V / R
Voltage
V = I · R
Power
P = VI = I²R = V²/R
Temperature Adjustment
R(T) ≈ Rref[1 + α(T−Tref)]
What This Resistor Impedance Calculator Offers
Ideal resistor impedance in rectangular and polar form
Resistance magnitude and 0° phase angle
Voltage, current and power calculations
Reverse solving for current or voltage
Tolerance minimum/maximum resistance
Temperature coefficient adjustment
Responsive native-canvas engineering chart
Common resistor reference table
Dynamic calculation steps and engineering interpretation
How to Use This Calculator
  1. Select Basic or Advanced mode.
  2. Select whether you want impedance, current, or voltage.
  3. Enter the resistance and select the correct unit.
  4. Enter voltage or current when required.
  5. In Advanced mode, optionally enter tolerance, TCR, and operating temperature.
  6. Click Calculate and review impedance, phase, electrical power, and engineering limits.
Important Engineering Notes
Ideal AC behavior — an ideal resistor has no reactive component, so voltage and current are in phase and Z = R + j0.
Temperature — real resistance changes with temperature according to the resistor's temperature coefficient.
Tolerance — the marked resistance is nominal; real manufactured values can be above or below nominal within the rated tolerance.
High frequency — real resistors can develop parasitic inductance and capacitance, so their impedance may stop being purely resistive at sufficiently high frequency.
References: standard AC steady-state circuit theory and Ohm's law as presented in texts such as Alexander & Sadiku, Fundamentals of Electric Circuits, and Hayt, Kemmerly & Durbin, Engineering Circuit Analysis. For real components, use the resistor manufacturer's datasheet for tolerance, TCR, voltage rating, power derating, pulse capability, and high-frequency behavior.
Engineering Disclaimer: This calculator is intended for education, preliminary design, and engineering estimation. Verify resistor power rating, working voltage, tolerance, TCR, derating, pulse capability, insulation, and manufacturer specifications before final design.
Vorzep · Resistor Impedance Analysis Tool

Variables

Symbol Quantity Unit
ZR Impedance of a resistor Ω
R Resistance Ω

What it means

In AC circuit analysis, the impedance of a resistor is simply its resistance Z_R = R, with no imaginary part. It is real and independent of frequency. This means that a resistor behaves exactly the same at DC and at all AC frequencies (ideally). The voltage and current are in phase (phase angle 0°). The magnitude of impedance is |Z_R| = R, and the power factor is unity. In phasor notation, the voltage across a resistor is in phase with the current. This simplicity makes resistors essential in AC circuits for limiting current and setting bias points. Example: A resistor of 100Ω has an impedance Z_R = 100∠0° Ω. At 50Hz or 1MHz, its impedance remains 100Ω, and there is no phase shift between voltage and current.

Worked example

Impedance of a Resistor – Practical Example

Real‑World
Scenario: A 100 Ω resistor is used in an AC circuit. What is its impedance at any frequency?
ParameterValue
R100 Ω
FormulaZR = R
1Impedance is just the resistance: ZR = 100 Ω (0° phase)
Final Design ZR = 100 Ω ∠0° ✓ Pure real
Why: Resistors have no reactive component – their impedance is frequency‑independent and purely resistive.

Common mistakes

  • Resistor impedance: Purely real – no phase shift between voltage and current.
  • Frequency independent: Ideal resistors have the same impedance at all frequencies.
  • Parasitic effects: Real resistors have inductance and capacitance at high frequencies – not captured.
  • Units: Ohms.

Applications

The impedance of a resistor is purely real and equal to R, independent of frequency. This means a resistor behaves identically at DC and all AC frequencies (ideally). Engineers use this to design resistive networks, to set bias points, and to create feedback networks. In AC analysis, the resistor's impedance is represented as a real number, simplifying phasor calculations. By understanding the frequency‑independent nature of resistance, professionals can design stable and predictable circuits. This formula is fundamental to both DC and AC circuit analysis and is essential for all electrical engineers.

  • Bias networks for amplifiers and active filters
  • Feedback and compensation networks
  • Power dissipation and thermal management
  • Voltage and current sensing circuits
  • Educational foundation of impedance concept

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  • Published July 10, 2026
  • Last updated August 15, 2026