Capacitive reactance is how strongly a capacitor opposes alternating current, in ohms: X_C = 1/(2πfC). A capacitor charges and discharges each cycle, and the faster it has to do that — higher frequency — the less it impedes the current. So unlike an inductor, its reactance falls as frequency or capacitance rises. That's what lets capacitors couple high-frequency signals through while blocking DC, and it's the inverse partner of inductive reactance in every AC and resonance calculation.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the capacitive reactance relation X_C = 1/(2πfC).
The capacitive reactance equations
Convert capacitance to farads first — microfarads are 10⁻⁶ F. The 2π converts the cyclic frequency to angular frequency ω, and the reactance is one over ωC. To find the frequency at which a capacitor reaches a given reactance, rearrange to f = 1/(2πC·X_C); to choose the capacitance for a target reactance, C = 1/(2πf·X_C). The reactance is the ideal AC opposition, distinct from any small series resistance or leakage.
Worked example — a coupling capacitor
Scenario: A 10 µF capacitor is used at 60 Hz mains, then in a 1 kHz audio path. What is its reactance in each case?
At mains the capacitor offers a sizeable 265 Ω, but at 1 kHz its reactance collapses to just 15.9 Ω — about 17 times lower, exactly tracking the 17× rise in frequency. At 50 Hz it would be 318 Ω. This falling reactance is precisely why a coupling capacitor passes the audio signal almost freely while still blocking the DC bias: high frequencies see a low impedance, DC sees an open circuit.
Frequently Asked Questions
X_C = 1/(2πfC). 10 µF at 60 Hz = 1/(2π × 60 × 10×10⁻⁶) = 265.3 Ω. Convert µF to F first.
The AC opposition of a capacitor in ohms. Falls as f or C rises, so caps pass high frequencies.
Inversely — double f, half X_C. 10 µF: 318 Ω at 50 Hz, 15.9 Ω at 1 kHz.
X_C = 1/(2πfC) falls with f; X_L = 2πfL rises with f. They cancel at resonance.
Infinite — at 0 Hz, X_C → ∞. A capacitor blocks DC once charged.