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Capacitive and Inductive Reactance Calculator

Reactance is the AC counterpart of resistance, but it depends on frequency. A capacitor blocks DC and passes high frequencies; an inductor does the opposite. Both are measured in ohms and neither dissipates power.

Reactance — X꜀ = 1/(2πfC), Xʟ = 2πfL

How much a capacitor or inductor opposes AC at a given frequency. Leave C or L blank if you only need one.

Capacitive reactance X꜀
Inductive reactance Xʟ
  1. 1.Given: f = 1 kHz, C = 1 µF, L = 10 mH
  2. 2.FormulaX꜀ = 1 ÷ (2πfC)SubstituteX꜀ = 1 ÷ (2π × 1000 × 1e-6)ResultX꜀ = 159.1549 Ω
  3. 3.FormulaXʟ = 2πfLSubstituteXʟ = 2π × 1000 × 0.01ResultXʟ = 62.8319 Ω
Common trap: Reactance is measured in ohms but it is not resistance — an ideal capacitor or inductor dissipates no power. And they move in opposite directions: X꜀ falls as frequency rises, Xʟ rises. That crossover point is resonance.

The formula

X_C = 1 / (2πfC) ; X_L = 2πfL

X_C
capacitive reactance (ohms)
X_L
inductive reactance (ohms)
f
frequency (hertz)
C
capacitance (farads)
L
inductance (henries)

Worked example

A 10 µF capacitor and a 100 mH inductor, both at 50 Hz.

  1. X_C = 1 / (2π × 50 × 10 × 10⁻⁶) = 1 / 0.0031416
  2. X_L = 2π × 50 × 0.1

X_C = 318.3 Ω and X_L = 31.4 Ω at mains frequency.

Where you'll use it

Filter design, power-factor work, and understanding why a motor's inrush differs from its running current. Because the two reactances move in opposite directions with frequency, there is always one frequency where they cancel — resonance.

The laws behind it

Parts this applies to

Capacitor (ceramic / MLCC)Capacitor (aluminium electrolytic)Capacitor (tantalum)Capacitor (film)SupercapacitorQuartz Crystal / ResonatorMOSFET555 Timer IC

Related calculators

RLC Impedance CalculatorZ = √(R² + (X_L − X_C)²)LC Resonance Calculatorf₀ = 1 / (2π√(LC))RC Time Constant Calculatorτ = R × CPower Factor Correction CalculatorQ_c = P × (tan φ₁ − tan φ₂), where φ = arccos(pf)