Inductive and capacitive reactance
Reactance is the frequency-dependent opposition an ideal inductor or capacitor presents to alternating current. It is measured in ohms.
Inductive reactance rises with frequency and inductance; capacitive reactance falls as frequency or capacitance rises.
How to calculate reactance
- Select component: Choose inductive or capacitive reactance.
- Enter frequency: Supply a positive AC frequency and unit.
- Enter L or C: Supply a positive component value and unit.
- Calculate: Generate reactance in ohms.
Formula and variables
Convert inputs to hertz, henries, or farads before applying the selected equation.
XL = 2πfL; XC = 1/(2πfC)- f — Frequency
- AC frequency (Hz)
- L — Inductance
- Ideal inductance (H)
- C — Capacitance
- Ideal capacitance (F)
Inductor at line frequency
Find XL for 100 mH at 60 Hz.
- f
- 60 Hz
- L
- 100 mH
- XL = 2π(60)(0.100)
Result: XL ≈ 37.70 Ω.
The ideal inductor presents about 37.7 ohms of reactance.
Understanding your results
Ideal-component result
Reactance is the imaginary component of impedance; resistance and parasitic effects are not included.
- Inductive reactance has a positive imaginary sign.
- Capacitive reactance has a negative imaginary sign in impedance notation.
Assumptions
- Sinusoidal steady-state operation.
- Ideal component values are constant at the selected frequency.
Limitations
- Does not include resistance, ESR, parasitics, tolerance, or resonance.
- Does not calculate total circuit impedance.
Common mistakes
- Entering millihenries as henries.
- Using frequency where angular frequency is expected without 2π.
- Confusing reactance with resistance.
Practical use cases
AC circuit study
Compare inductor and capacitor behavior across frequency.
Frequently asked questions
What unit is reactance measured in?
Ohms, the same unit used for resistance and impedance.
Why does capacitive reactance fall with frequency?
The inverse relationship XC = 1/(2πfC) makes XC smaller as f increases.
Sources and review
- Simple AC Circuits — OpenStax University Physics Volume 2. Accessed 2026-07-13.
Reviewed 2026-07-13.