Reactance Calculator

Inductive and capacitive reactance

Operating Mode

Required Parameters

Frequency

Capacitance

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Quick Answer

Capacitive reactance: Xc = 1/(2πfC) — decreases with frequency. Inductive reactance: XL = 2πfL — increases with frequency. Both measured in ohms and combine with resistance to form impedance.

Documentation

Reactance Calculator — Engineering Reference

Use this calculator to compute capacitive reactance (Xc) and inductive reactance (XL) at any frequency. Reactance is the opposition to alternating current by capacitors and inductors.

Capacitive Reactance

Xc = 1 / (2pi x f x C)

Where:

Xc = Capacitive reactance (ohms)
f = Frequency (Hz)
C = Capacitance (Farads)

Capacitive reactance decreases as frequency increases — capacitors pass high frequencies more easily.

Inductive Reactance

XL = 2pi x f x L

Where:

XL = Inductive reactance (ohms)
f = Frequency (Hz)
L = Inductance (Henrys)

Inductive reactance increases with frequency — inductors block high frequencies.

Impedance

Total impedance combines resistance and reactance:

Z = sqrt(R squared + X squared)

Where X = XL - Xc (net reactance).

Resonance

At resonance, XL = Xc, and impedance is purely resistive:

f0 = 1 / (2pi x sqrt(L x C))

Quick Reference Table

Component	1 kHz	10 kHz	100 kHz	1 MHz
100 nF cap	1.59 kOhm	159 Ohm	15.9 Ohm	1.59 Ohm
1 uF cap	159 Ohm	15.9 Ohm	1.59 Ohm	0.16 Ohm
100 uH ind	0.63 Ohm	6.28 Ohm	62.8 Ohm	628 Ohm
1 mH ind	6.28 Ohm	62.8 Ohm	628 Ohm	6.28 kOhm

Applications

Filter design — Select components for low-pass, high-pass, and band-pass filters
Impedance matching — Match source and load impedances for maximum power transfer
Power factor correction — Size capacitor banks to compensate inductive loads
Tuned circuits — Design LC resonant circuits for radio and communications

Related Tools

Filter Cutoff Calculator — Design active and passive filters
Trace Impedance Calculator — Calculate transmission line impedance
Ohm's Law Calculator — Basic V, I, R, P relationships

Design Notes

Reactance is the key to understanding AC behavior of passive components. Capacitors look like open circuits at DC but short circuits at high frequency. Inductors are the opposite. Where capacitive and inductive reactances are equal (XL = Xc), a resonant circuit forms with impedance that is purely resistive. Understanding reactance helps you select the right capacitor for decoupling, the right inductor for filtering, and design impedance matching networks.

Common Mistakes

1
Treating a capacitor as a simple open or short circuit — its behavior depends entirely on frequency. A 100nF cap is 1.6MΩ at 1 Hz but only 1.6Ω at 1 MHz.
2
Ignoring parasitic inductance (ESL) in capacitors which causes them to become inductive above their self-resonant frequency.
3
Confusing reactance (X, imaginary) with impedance (Z, complex). Impedance includes both resistance and reactance: Z = √(R² + X²).

Engineering Handbox

1. Identify: f = 1000 Hz, C = 100 × 10⁻⁹ F 2. Xc = 1 / (2π × 1000 × 100 × 10⁻⁹) 3. Xc = 1 / (6.283 × 10⁻⁴) 4. Xc = 1591.5 Ω

VerificationThe capacitive reactance at 1 kHz is 1591.5 Ω (1.59 kΩ). At 10 kHz it drops to 159 Ω.

Knowledge Base

What is the formula for capacitive reactance?

Xc = 1 / (2π × f × C), where Xc is in ohms, f is frequency in Hz, and C is capacitance in farads. Capacitive reactance DECREASES with frequency — a 1µF capacitor has 159Ω at 1 kHz but only 0.16Ω at 1 MHz. At DC (f=0), Xc is infinite (open circuit).

What is the formula for inductive reactance?

XL = 2π × f × L, where XL is in ohms, f is frequency in Hz, and L is inductance in henries. Inductive reactance INCREASES with frequency — a 10mH inductor has 62.8Ω at 1 kHz and 62,832Ω at 1 MHz. At DC (f=0), XL is zero (short circuit).

What is the difference between reactance and resistance?

Resistance (R) dissipates energy as heat and is frequency-independent. Reactance (X) stores energy in electric fields (capacitors) or magnetic fields (inductors) and is frequency-dependent. Both are measured in ohms. Combined, they form impedance: Z = R + jX.

What is impedance vs reactance?

Impedance (Z) is the total opposition to AC current: Z = √(R² + X²) in magnitude. It has both real (R) and imaginary (X) parts: Z = R + jX. Reactance is just the imaginary part. A pure resistor has Z = R. A pure capacitor has Z = -jXc. A series RC circuit has Z = R - jXc.

At what frequency does a capacitor look like a short circuit?

When Xc << R_load (typically Xc < R/10). For a 100nF cap feeding a 10kΩ load: Xc = R/10 at f = 1/(2π × 100nF × 1kΩ) = 1.6 kHz. Above this frequency, the capacitor effectively passes the signal. This is the basis of AC coupling / high-pass filter design.

What is the self-resonant frequency (SRF) of a capacitor?

Every real capacitor has parasitic inductance (ESL) from its leads and internal structure. At the SRF, XL = Xc and the capacitor resonates, acting as a near-short-circuit. Above the SRF, the capacitor actually behaves as an inductor. Typical SRFs: through-hole electrolytics ~1 MHz, 0805 MLCCs ~100 MHz, 0201 MLCCs ~1 GHz.

How do I calculate the resonant frequency of an LC circuit?

f_resonant = 1 / (2π × √(L × C)). At resonance, XL = Xc and the impedance is purely resistive (minimum for series LC, maximum for parallel LC). Example: 10µH and 100pF resonate at 1/(2π × √(10µH × 100pF)) = 5.03 MHz.

Why does a capacitor block DC but pass AC?

At DC (f = 0 Hz), Xc = 1/(2π × 0 × C) = infinite ohms — no current flows. As frequency increases, Xc decreases, allowing more current. Physically, AC alternately charges and discharges the capacitor, creating a continuous alternating current through the circuit even though no electrons pass through the dielectric.

Why does an inductor pass DC but block AC?

At DC (f = 0 Hz), XL = 2π × 0 × L = 0 ohms — the inductor is just a piece of wire. As frequency increases, XL increases, opposing current changes more aggressively. This is why inductors are used as chokes in power supply filtering — they pass DC power while blocking high-frequency noise.

How do I use reactance in circuit design?

Common uses: (1) Calculate capacitor size for AC coupling (set Xc << R_load at signal frequency). (2) Design LC filters (set resonance at desired cutoff). (3) Calculate inductor for power supply choke (set XL >> R at switching frequency). (4) Impedance matching networks for RF. (5) Determine capacitor adequacy for decoupling at operating frequency.

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