reactance calculator

What Is Reactance?

Reactance is the opposition that inductors and capacitors present to alternating current (AC). It is similar to resistance, but unlike resistance, reactance depends on frequency. This is why AC circuit analysis always asks two important questions: what is the component value, and at what frequency?

Inductive vs Capacitive Reactance

Inductive Reactance (X_L)

Inductors oppose changes in current. Their reactance increases as frequency increases. The formula is:

X_L = 2πfL

• f = frequency in hertz (Hz)
• L = inductance in henries (H)
• X_L = inductive reactance in ohms (Ω)

Capacitive Reactance (X_C)

Capacitors oppose changes in voltage. Their reactance decreases as frequency increases. The formula is:

X_C = 1 / (2πfC)

• f = frequency in hertz (Hz)
• C = capacitance in farads (F)
• X_C = capacitive reactance in ohms (Ω)

How to Use This Reactance Calculator

1. Select whether you want inductive or capacitive reactance.
2. Enter the signal frequency and choose the unit (Hz, kHz, MHz, or GHz).
3. Enter your inductor or capacitor value and pick the correct unit.
4. Click Calculate Reactance to get your result instantly.

Worked Examples

Example 1: Inductor at 1 kHz

Suppose L = 10 mH and f = 1 kHz. Converting units gives L = 0.01 H and f = 1000 Hz.

X_L = 2π(1000)(0.01) ≈ 62.83 Ω

Example 2: Capacitor at 60 Hz

Suppose C = 47 µF and f = 60 Hz. Converting units gives C = 47 × 10^-6 F.

X_C = 1 / [2π(60)(47×10^-6)] ≈ 56.45 Ω

Why Frequency Matters So Much

The same component behaves very differently at different frequencies. A capacitor that acts almost like an open circuit at low frequency may look like a very small impedance at radio frequencies. Similarly, an inductor may be nearly invisible at low frequency but strongly block AC at high frequency.

• Higher frequency → higher X_L
• Higher frequency → lower X_C

Practical Uses

Reactance calculations are useful in many real-world electrical and electronics tasks:

• Designing filters (low-pass, high-pass, band-pass)
• Sizing components in power supplies
• Understanding motor and transformer behavior
• RF tuning and impedance matching
• Analyzing AC load characteristics

Common Mistakes to Avoid

• Mixing units (for example, entering µF as if it were F).
• Using DC frequency (0 Hz) in formulas intended for AC reactance.
• Confusing reactance (Ω) with resistance (Ω)—same unit, different physics.
• Ignoring significant digits when values span from pF to mF or Hz to GHz.

Quick Reference

Unit Conversions

• 1 kHz = 1,000 Hz
• 1 MHz = 1,000,000 Hz
• 1 mH = 0.001 H
• 1 µH = 0.000001 H
• 1 µF = 0.000001 F
• 1 nF = 0.000000001 F
• 1 pF = 0.000000000001 F

Final Thoughts

A good reactance calculator saves time and prevents conversion errors. Whether you're a student solving homework problems or an engineer checking a design, fast and accurate reactance values help you make better circuit decisions.