Inductive Reactance Calculator — XL = 2πfL

Background.

Put a 100 mH coil in series with a 600 Ω audio line and the result is lopsided. At 50 Hz the coil presents 31.4 Ω and the line hardly notices it; at 1 kHz it presents 628.3 Ω and only 69% of the signal voltage reaches the load. Nothing about the coil has changed between the two cases. Its opposition to alternating current, its inductive reactance, simply grows in step with frequency.

The relationship is X_L = 2πfL: frequency in hertz times inductance in henries times 2π gives ohms. It is a straight line through zero, so a coil is effectively a short circuit to steady direct current and an increasingly stiff obstacle as the frequency climbs, the reverse of a capacitor. Only the product of frequency and inductance matters, which is why 100 mH at 1 kHz, 10 µH at 10 MHz and 100 nH at 1 GHz all come to the same 628.32 Ω.

Design work usually runs the equation backwards, so it can be solved three ways: the reactance of a known coil at a known frequency, the inductance needed to reach a target reactance, or the frequency at which a coil you already have reaches a given reactance. Each answer comes with the angular frequency and the susceptance, 1/X_L, the form that adds directly when parts sit in parallel.

All of this describes an ideal coil. A real one has resistance in its winding, capacitance between its turns and, often, a magnetic core that saturates under heavy current, and each of these sets a limit on where the straight line still holds.

What is inductive reactance calculator?

Inductive reactance is the opposition an ideal inductor offers to a sinusoidal current, measured in ohms like resistance but working differently: instead of turning energy into heat, the coil stores it in its magnetic field for part of each cycle and hands it back for the rest. In complex notation the inductor's impedance is +jX_L, meaning the voltage across it leads the current through it by exactly a quarter cycle, 90°.

How to use this calculator.

1. 01Choose what to solve for: reactance from frequency and inductance, inductance from frequency and reactance, or frequency from inductance and reactance.
2. 02Enter the frequency and pick its unit, from Hz to GHz. Mains hum is 50 Hz or 60 Hz depending on the country; for a switching supply use its switching frequency.
3. 03Enter the inductance with its unit, nH to H, using the part's marked nominal value; most inductors carry a tolerance, so a 100 mH part may not measure exactly 100 mH.
4. 04In the two inverse modes, enter the target reactance as a positive number in Ω, kΩ or MΩ. Whichever field the chosen mode does not need is ignored.
5. 05For high frequencies or heavy currents, compare the answer with the datasheet's self-resonant frequency and saturation current before relying on it.

The formula.

The equation follows from Faraday's law, which makes the voltage across a coil equal to L times the rate of change of its current, v = L·di/dt. Drive it with i = I₀ sin(ωt) and the voltage is v = ωL·I₀ cos(ωt): its amplitude is ωL times the current amplitude, and the cosine runs a quarter cycle ahead of the sine, which is the 90° lead. The ratio ωL is the reactance, written X_L = ωL in physics texts and X_L = 2πfL in engineering, since ω = 2πf. The other two modes are rearrangements, L = X_L ÷ (2πf) and f = X_L ÷ (2πL), and susceptance is B = 1 ÷ X_L in siemens. Because voltage and current are 90° apart, the average power ½I₀V₀cos φ is zero for an ideal inductor. Every input must be positive: at 0 Hz the reactance is zero and the inverse modes would divide by nothing. Not modelled are winding resistance, core loss, skin effect, self-resonance and core saturation.

A worked example.

Frequently asked questions.

References& sources.

1. [1]OpenStax (Rice University), University Physics Volume 2, §15.3 'RLC Series Circuits with AC'. Source of the defining equation used on this page: Eq. 15.8, X_L = ωL. Open access; retrieved 2026-07-29.
2. [2]James M. Fiore, AC Electrical Circuit Analysis: A Practical Approach, §8.2 'Series Resonance' (Engineering LibreTexts). Cited for the 2πf engineering convention used throughout this page, and for the statement that inductive and capacitive reactance cancel at resonance — Eq. 8.2 f₀ = 1/(2π√LC) and Eq. 8.12 Q_series = X₀/R_T. Open access; retrieved 2026-07-29.
3. [3]OpenStax, University Physics Volume 2, §15.4 'Power in an AC Circuit'. Source of the average-power relation P_ave = ½I₀V₀cos φ (Eq. 15.12) and the identification of cos φ = R/Z as the power factor — together these are why an ideal inductor, at φ = 90°, dissipates no average power. Open access; retrieved 2026-07-29.
4. [4]IEC 60050, International Electrotechnical Vocabulary (Electropedia), entry 131-12-46 'reactance', X = Im(Z) — the normative source for the terminology. ACCESS NOTE: electropedia.org returns HTTP 403 to automated retrieval, so the entry number and term were confirmed from the IEC's indexed entry title but the definition text was not opened. No figure on this page is taken from it.