Reactance is the opposition that capacitors and inductors present to alternating current. Capacitive reactance decreases as frequency increases, while inductive reactance increases as frequency increases.

What is the formula for capacitive reactance?

Capacitive reactance is Xc = 1 / (2πfC), where f is frequency in hertz and C is capacitance in farads.

What is the formula for inductive reactance?

Inductive reactance is Xl = 2πfL, where f is frequency in hertz and L is inductance in henries.

What happens when XL equals XC?

When inductive reactance equals capacitive reactance, the circuit is at resonance. In a series RLC circuit, the reactive parts cancel and impedance is mainly resistive.

Reactance Calculator — Capacitive & Inductive Reactance

Reactance Calculator

Calculate capacitive reactance, inductive reactance, total RLC impedance, resonance frequency and phase angle for AC circuits.

⚡ Key Insight: Capacitors oppose low frequency and pass high frequency. Inductors oppose high frequency and pass low frequency. At resonance, XL = XC, so the reactive part cancels and a series RLC circuit becomes mostly resistive.

〰️ Reactance — Frequency Dependence

Capacitance

Frequency

AC Voltage (optional)

Presets: 100µF @ 50Hz10µF @ 60Hz100nF @ 1kHz1nF @ 100kHz22pF @ 10MHz

Capacitive Reactance

—

Capacitor

🧠 Behavior: Lower capacitance or lower frequency gives higher Xc. Higher frequency makes a capacitor easier for AC to pass.

Frequency

—

converted to Hz

Capacitance

—

converted to farads

Current @ V

—

I = V / Xc

Admittance

—

1 / Xc

Phase

-90°

current leads voltage

Formula

1/(2πfC)

capacitive

———

Inductance

Frequency

AC Voltage (optional)

Presets: 10mH @ 1kHz1H @ 50Hz100µH @ 100kHz47µH @ 1MHz5mH @ 60Hz

Inductive Reactance

—

Inductor

🧠 Behavior: Higher inductance or higher frequency gives higher Xl. Inductors increasingly block AC as frequency rises.

Frequency

—

converted to Hz

Inductance

—

converted to henries

Current @ V

—

I = V / Xl

Admittance

—

1 / Xl

Phase

+90°

current lags voltage

Formula

2πfL

inductive

———

Resistance

Inductance

Capacitance

Frequency

AC Voltage (optional)

Presets:Audio filterSMPS tankRF exampleSpeaker crossover

Series RLC Impedance

—

2πfL

—

1/(2πfC)

Net Reactance

—

X = XL − XC

Phase Angle

—

atan(X/R)

Current @ V

—

I = V / |Z|

Resonance f₀

—

1/(2π√LC)

———

Inductance

Capacitance

Resistance / ESR (optional)

Presets:10mH + 100nF100µH + 100nFCrossoverRF LC tank

Resonant Frequency

—

XL = XC

🎯 At resonance: Inductive reactance equals capacitive reactance. In a series RLC circuit, impedance is minimum and mostly equal to resistance.

XL at f₀

—

same as XC

XC at f₀

—

same as XL

Q Factor

—

if R entered

Bandwidth

—

f₀ / Q

Angular Frequency

—

ω₀ = 2πf₀

Formula

1/(2π√LC)

LC resonance

———

📐 Reactance Formulas

Capacitor

XC = 1 / (2πfC)
I = V / XC
Phase = -90°
Current leads voltage

Inductor

XL = 2πfL
I = V / XL
Phase = +90°
Current lags voltage

Series RLC

X = XL − XC
|Z| = √(R² + X²)
θ = atan(X/R)
I = V / |Z|

Resonance

f₀ = 1/(2π√LC)
XL = XC at f₀
Q = XL / R
BW = f₀ / Q

📋 Quick Reference

🟠 Common Capacitors

100 nF @ 1 kHz1.59 kΩ

1 µF @ 1 kHz159 Ω

10 µF @ 50 Hz318 Ω

100 µF @ 50 Hz31.8 Ω

🔵 Common Inductors

10 µH @ 100 kHz6.28 Ω

100 µH @ 100 kHz62.8 Ω

10 mH @ 1 kHz62.8 Ω

1 H @ 50 Hz314 Ω

🎯 Useful Frequencies

Mains50 / 60 Hz

Audio midband1 kHz

SMPS range50–500 kHz

RF / crystalMHz

📚 Engineering Notes

Capacitors block DC but pass ACAt 0 Hz, XC becomes infinite. As frequency rises, XC falls, which is why capacitors are used for coupling, bypassing and filtering high-frequency noise.

Inductors pass DC but oppose ACAt 0 Hz, XL is nearly zero except winding resistance. As frequency rises, XL increases, so inductors are used for chokes, filters and energy storage in switching circuits.

Reactance is not resistanceReactance stores and releases energy instead of dissipating it as heat. It still limits AC current, but the voltage/current phase relationship changes.

Series vs parallel RLC mattersThis calculator uses the common series RLC impedance formula. Parallel RLC has a different impedance behavior and is often used for tuned RF circuits.

What is Reactance?

Reactance is the opposition to AC current caused by capacitors and inductors. It is measured in ohms, like resistance, but it depends strongly on frequency and causes a phase shift between voltage and current.

Capacitive Reactance vs Inductive Reactance

Capacitive reactance uses XC = 1/(2πfC), so it decreases when frequency or capacitance increases. Inductive reactance uses XL = 2πfL, so it increases when frequency or inductance increases.

❓ Frequently Asked Questions

Reactance is measured in ohms (Ω), just like resistance. The difference is that reactance depends on frequency and shifts phase, while ideal resistance does not.

A capacitor charges and discharges each AC cycle. At higher frequency, it cycles faster, so more AC current can pass for the same voltage. That makes the effective opposition, XC, lower.

An inductor resists change in current. At higher frequency, current changes faster, so the inductor produces a larger opposing voltage. That makes XL higher.

That condition is resonance. In a series RLC circuit, the inductive and capacitive reactances cancel, leaving mainly resistance. Current is highest if resistance is low.

💡 Rule of Thumb: For a quick mental check, a 1 µF capacitor has about 159 Ω reactance at 1 kHz. A 10 mH inductor has about 63 Ω reactance at 1 kHz.