Calculator Inputs
Example Data Table
| Case | R | f | L | C | Voltage | Expected Use |
|---|---|---|---|---|---|---|
| DC sensing resistor | 10 Ω | 0 Hz | 5 nH | 0.1 pF | 2 V | Near-pure resistance check |
| General signal path | 1 kΩ | 100 kHz | 10 nH | 0.2 pF | 5 V | Routine impedance estimation |
| RF bias resistor | 50 Ω | 100 MHz | 2 nH | 0.5 pF | 1 V | High-frequency parasitic study |
| Precision divider leg | 100 kΩ | 10 kHz | 20 nH | 0.05 pF | 10 V | Tolerance and loading review |
Formula Used
An ideal resistor has no reactive part, so its impedance is Z = R.
For a practical resistor, this page uses a common high-frequency model:
ZRL = R + jωL
ZC = 1 / (jωC) = -j / (ωC)
Zeq = (ZRL × ZC) / (ZRL + ZC)
Where ω = 2πf, R is resistance, L is parasitic inductance, and C is parasitic capacitance.
The calculator also reports:
|Z| = √(Re(Z)² + Im(Z)²)Phase = tan⁻¹(Im(Z) / Re(Z))I = V / |Z|S = V × IP = S × cos(φ)Q = S × sin(φ)fr = 1 / (2π√LC)when both parasitics exist
How to Use This Calculator
- Choose the ideal model or the practical parasitic model.
- Enter resistance and select the correct unit.
- Enter operating frequency and its unit.
- Provide RMS voltage to estimate current and power.
- Add parasitic inductance and capacitance for high-frequency work.
- Enter tolerance to see impedance spread around nominal value.
- Click the calculate button.
- Review the result panel above the form and export CSV or PDF when needed.
Frequently Asked Questions
1. Why does an ideal resistor show zero phase angle?
An ideal resistor stores no electric or magnetic energy. Voltage and current stay in phase, so the impedance contains only a real component and the phase angle remains zero degrees.
2. Why include parasitic inductance and capacitance?
Real resistors have leads, geometry, and internal structure that add small inductance and capacitance. At higher frequencies, these parasitics can noticeably change impedance magnitude, phase, and resonance behavior.
3. What happens at zero frequency?
At DC, inductive reactance becomes zero and capacitive reactance becomes extremely large. The practical model therefore behaves almost like a pure resistor, assuming no unusual leakage paths are involved.
4. What does the resonant frequency mean here?
It estimates where the parasitic inductance and capacitance interact most strongly. Around that frequency, a practical resistor may stop behaving like a simple resistance and can show sharper phase and magnitude changes.
5. Why can the imaginary part be positive or negative?
A positive imaginary part indicates net inductive behavior, while a negative imaginary part indicates net capacitive behavior. The sign shows whether the component leads or lags current at the chosen frequency.
6. How is tolerance used in this page?
Tolerance changes the nominal resistance into minimum and maximum resistance values. The calculator recomputes impedance magnitude using those limits, helping you estimate spread caused by resistor manufacturing variation.
7. Is this suitable for RF design work?
It is useful for first-pass analysis and quick comparison. For final RF validation, use measured S-parameters, manufacturer models, layout extraction, and full circuit simulation because package and board effects can dominate.
8. What graph does the page generate?
The chart sweeps frequency around your selected operating point and plots impedance magnitude with phase angle. This makes it easier to spot flat resistive regions, inductive rise, capacitive rolloff, or resonance shifts.