What is a reactance calculator?
In AC circuits, capacitors and inductors do not behave like simple resistors. Instead, they oppose current flow in a way that depends on frequency. This opposition is called reactance. A reactance calculator helps you determine:
- Capacitive reactance (XC)
- Inductive reactance (XL)
- How a component behaves at different frequencies
This is essential for designing filters, tuning circuits, motor drives, PLC analog inputs, and any system involving AC waveforms or switching signals.
Capacitive vs inductive reactance
Capacitors and inductors react differently to changes in frequency:
| Component | Behavior | Reactance formula | Effect of frequency |
|---|---|---|---|
| Capacitor | Opposes voltage changes | XC = 1 / (2π f C) | Higher frequency → lower reactance |
| Inductor | Opposes current changes | XL = 2π f L | Higher frequency → higher reactance |
These relationships determine whether a component behaves more like a short circuit, an open circuit, or something in between.
Understanding AC impedance
Reactance is a crucial part of impedance, which is the total opposition to AC current and includes both resistance (R) and reactance (X). Depending on the sign of reactance:
- Positive reactance → inductive behavior
- Negative reactance → capacitive behavior
The full impedance of a component or network can be represented as a complex number: Z = R ± jX.
Key formulas used by this calculator
Capacitive reactance
XC = 1 / ( 2π f C )
Units: ohms (Ω)
As frequency increases, the capacitor's reactance drops, allowing more AC current to pass.
Inductive reactance
XL = 2π f L
Units: ohms (Ω)
As frequency increases, the inductor’s reactance rises, restricting AC current more strongly.
Practical examples
- Filtering: At low frequencies, capacitors block little current (high reactance), but at high frequencies they pass more (low reactance). This is the basis of high-pass filters.
- Motor control: Inductive reactance affects the current drawn by AC motors and transformers, influencing torque and power factor.
- Analog inputs and instrumentation: Reactance determines how sensors and PLC inputs behave with AC or switching signals, impacting accuracy and loading.
- Resonant circuits: Reactance determines resonance when XL = XC, allowing you to compute resonant frequency.
Design considerations when working with reactance
- Units matter: A small error in capacitance (nF vs µF) or inductance (µH vs mH) can change reactance by factors of 10–100.
- Component tolerances: Inductors vary with core material and load; capacitors drift with voltage, temperature and age.
- Frequency accuracy: Reactance calculations assume stable input frequency; power-line, PWM and switching waveforms may vary.
- High-frequency behavior: At RF or high-speed digital frequencies, parasitic capacitance and inductance can dominate ideal reactance.
This calculator gives a fast and accurate starting point for AC analysis in power systems, filters, communications, and control electronics. For complex networks, combine this with impedance solvers or circuit simulations for full accuracy.
