Inductive Reactance Calculator
Inductive reactance XL = 2πfL for any inductance and frequency — with current at a given voltage, impedance including winding resistance, quality factor and a frequency sweep.
How it works
XL = 2π f L; |Z| = √(R² + XL²); φ = atan(XL / R); Q = XL / R
An inductor resists changes in current: as alternating current tries to reverse, the coil’s magnetic field induces a back-EMF that opposes it, and the faster the reversal the stronger the opposition. Inductive reactance therefore grows in proportion to frequency — the mirror image of a capacitor — so inductors pass DC freely and choke high frequencies, which is what makes them filters and RF chokes. Real coils also have winding resistance, so the total impedance is the vector sum and the current lags the voltage by less than the ideal 90°; the ratio of reactance to resistance is the quality factor that determines how sharp a resonant circuit built from it will be.
Worked example
100 mH at 1 kHz: XL = 2π × 1000 × 0.1 = 628 Ω; with 2 Ω of wire resistance |Z| ≈ 628 Ω, Q = 314, and 12 V drives 19 mA lagging by 89.8°.
Frequently asked questions
Why does a choke get hot?
From I²R in the winding and from core losses (hysteresis and eddy currents) that rise with frequency — neither appears in the ideal reactance formula.
How do I pick an inductor for a low-pass filter?
Choose L so that XL at the cut-off frequency equals the load resistance (first order) — e.g. 8 Ω at 2.5 kHz needs 0.51 mH.
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This calculator is for educational purposes. Double-check important results before acting on them.