Inductive reactance is how strongly an inductor opposes alternating current, measured in ohms: X_L = 2πfL. Because a changing current induces a back-EMF that fights the change, the opposition grows with both how fast the current alternates (frequency) and how much inductance there is. That's why inductors act as chokes — barely noticed at DC and low frequencies, but increasingly blocking as the frequency climbs. It is the mirror image of a capacitor, whose reactance does the opposite.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the inductive reactance relation X_L = 2πfL.
The inductive reactance equations
Convert inductance to henries first — millihenries are 10⁻³ H. The factor 2π turns the cyclic frequency in hertz into the angular frequency ω in radians per second, and the reactance is simply ωL. To find the frequency at which an inductor reaches a given reactance, rearrange to f = X_L/(2πL); to size the inductance for a target reactance, L = X_L/(2πf). Reactance is purely the AC opposition, distinct from the wire's resistance.
Worked example — a choke at mains and audio
Scenario: A 10 mH inductor is used at 60 Hz mains, then in a 1 kHz audio circuit. What is its reactance in each case?
At mains frequency the inductor barely impedes the current with 3.77 Ω, but at 1 kHz its reactance jumps to 62.83 Ω — about 17 times higher, exactly tracking the 17× rise in frequency. At 50 Hz it would read 3.14 Ω. This steep rise with frequency is what lets an inductor pass the DC and low-frequency current a circuit needs while choking off high-frequency noise and ripple.
Frequently Asked Questions
X_L = 2πfL. 10 mH at 60 Hz = 2π × 60 × 0.01 = 3.77 Ω. Convert mH to H first.
The AC opposition of an inductor in ohms, from the back-EMF resisting current change. Rises with f and L.
Directly proportional. Double f → double X_L. 10 mH: 3.14 Ω at 50 Hz, 62.8 Ω at 1 kHz.
X_L = 2πfL rises with f; X_C = 1/(2πfC) falls with f. They cancel at resonance.
Zero — at 0 Hz, X_L = 0. An inductor passes DC with only its winding resistance.