Reactance calculator
Capacitor and inductor reactance at a frequency, and the impedance and phase of a series RC, RL or RLC circuit.
Capacitive reactance X_C
159.2Ω
Current leads voltage by 90°.Reactance by decade for 100 nF
| Frequency | X_C |
|---|---|
| 10 Hz | 159.2 kΩ |
| 100 Hz | 15.92 kΩ |
| 1 kHz | 1.592 kΩ |
| 10 kHz | 159.2 Ω |
| 100 kHz | 15.92 Ω |
| 1 MHz | 1.592 Ω |
| 10 MHz | 159.2 mΩ |
| 100 MHz | 15.92 mΩ |
| 1 GHz | 1.592 mΩ |
Method
How this is calculated
X_C = 1 / ( 2π · f · C ) X_L = 2π · f · L |Z| = sqrt( R² + (X_L − X_C)² ) φ = atan( (X_L − X_C) / R ) f0 = 1 / ( 2π · sqrt( L · C ) )
- Sign
- Positive phase is inductive (current lags voltage); negative is capacitive (current leads).
- Ideal parts
- Real capacitors have ESR and ESL, so above their self-resonant frequency a capacitor behaves as an inductor. Real inductors have winding resistance and capacitance. Use the part datasheet impedance curve near those limits.
FAQ
Questions
At 10 kHz, 159.2 Ω. At 1 MHz, 1.59 Ω. At 100 MHz an ideal 100 nF would be 0.016 Ω, but a real part is usually past its self-resonant frequency there, so read its impedance curve in the datasheet.
2π × 1 MHz × 10 µH = 62.8 Ω. Reactance rises in proportion to frequency: 628 Ω at 10 MHz.
As vectors, not by adding: |Z| = sqrt(R² + X²). 100 Ω in series with 100 Ω of capacitive reactance is 141.4 Ω at −45°.
X_L equals X_C, they cancel and the impedance falls to R alone. 100 µH and 100 nF resonate at 50.33 kHz.
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