Inductive Reactance Calculator — XL = 2πfL

Background.

Inductive reactance is the opposition an inductor presents to alternating current, measured in ohms and rising in direct proportion to frequency. It is the exact mirror of capacitive reactance: where a capacitor blocks DC and passes high frequencies, an inductor passes DC almost freely and blocks high frequencies more and more strongly. That single fact is behind chokes, RF filters, transformer magnetising current, and the reason a length of wire that is invisible at 50 Hz becomes a real circuit element at a gigahertz.

The equation is XL = 2πfL, with frequency in hertz and inductance in henries, giving ohms. This calculator runs it in all three directions, because sizing work usually goes backwards. Choose reactance and you get ohms from a frequency and a part value. Choose inductance and you get the henries needed to reach a target impedance at a chosen frequency — the calculation behind sizing a common-mode choke or a supply-line filter. Choose frequency and you find where an inductor you already have reaches a given reactance, which is the calculation behind a filter corner or a matching decision.

One convention, stated plainly. Reactance is shown here as a positive magnitude. In complex notation an inductor's impedance is plus j times XL, which means the voltage across it leads the current through it by exactly 90 degrees — the opposite of a capacitor, where current leads. Because the phase angle is 90 degrees, an ideal inductor dissipates no average power at all; it stores energy in its magnetic field and returns it every cycle. Any heat in a real inductor comes from its winding resistance and its core, never from its reactance.

The susceptance output is the reciprocal, B equals 1 over XL, in siemens. It is the form to use for parallel networks, where susceptances add directly while reactances do not.

Two limits belong beside the answer rather than in a footnote, because both of them break this equation completely rather than just bending it. The first is self-resonance. Every real inductor has capacitance between its turns, and above the frequency where that capacitance resonates with the inductance the part stops being an inductor: its impedance peaks and then falls with frequency, exactly the reverse of what XL = 2πfL predicts. Manufacturers publish a self-resonant frequency for this reason, and above it the calculation on this page describes a component you do not have.

The second is core saturation, and it is worse because this page cannot see it at all. A ferrite or powdered-iron core loses permeability as the flux in it approaches saturation, and the inductance falls with it. Since flux rises with current, the inductance collapses at exactly the moment the current is highest — which in a filter or a choke is exactly the moment you were relying on it. There is no current input on this page, so nothing here can warn you. Check the part's saturation current against your peak current, not just its inductance against your frequency.

What is inductive reactance calculator?

Inductive reactance, XL, is the magnitude of the opposition an ideal inductor offers to a sinusoidal current at a given frequency. It carries the unit of the ohm and relates the voltage across the component to the current through it, but unlike resistance it stores energy rather than dissipating it. Its value is XL = ωL = 2πfL, so it is directly proportional to both frequency and inductance: doubling either doubles the reactance. At DC, where f is zero, an ideal inductor's reactance is zero and it behaves as a plain wire — which is why an inductor passes DC and blocks AC, the mirror of a capacitor. In complex impedance notation the inductor contributes +jωL, and the positive imaginary sign encodes the phase: voltage leads current by 90 degrees. The physical origin is Faraday's law. A changing current through a coil changes the flux linking it, and the changing flux induces a voltage that opposes the change; the faster the current changes, the larger that opposing voltage, so a higher frequency produces a larger voltage for the same current amplitude — which is the definition of a larger impedance. The reciprocal quantity, susceptance B = 1/XL, is measured in siemens and is the form that adds directly across parallel branches. Inductive reactance is the property behind chokes, RF filters, tuned circuits, transformer magnetising current, and the parasitic impedance of every length of wire and every via at high frequency.

How to use this calculator.

1. 01Pick what you want to solve for. Reactance is the forward direction; inductance and frequency are the two inverses of the same equation.
2. 02Enter the two known quantities with their units. The third field is ignored in that mode, so a leftover value there will not be rejected.
3. 03Read the reactance in ohms and compare it against the impedance it works against. A choke is usually sized so its reactance at the frequency you want to block is many times the impedance of the path it is protecting.
4. 04Use the inductance mode to size a part: enter the frequency and the reactance you want, and the calculator returns the henries required.
5. 05Use the frequency mode to find where an inductor you already have reaches a given reactance — a filter corner, or the point where a parasitic starts to matter.
6. 06Read the susceptance when working with inductors in parallel; susceptances add directly, reactances do not.
7. 07Check two things on the datasheet before trusting the number: the self-resonant frequency, above which the part behaves capacitively and this equation is simply wrong, and the saturation current, which this page cannot see because it has no current input.

The formula.

The equation. Faraday's law says the voltage across an inductor is L times the rate of change of current. For a sinusoid at angular frequency ω that derivative introduces a factor of ω, so the ratio of voltage amplitude to current amplitude is ωL — the reactance. OpenStax's University Physics Volume 2 prints it at §15.3 as Eq. 15.8, X_L = ωL. Substituting ω = 2πf gives the engineering form used throughout this page.

The three modes are the same equation rearranged, and they are exact inverses. Solve for reactance and feed the answer back in inductance mode with the same frequency, and the original inductance comes back; do the same in frequency mode and the original frequency comes back. Both round trips are asserted as tests, once at the worked example and once at 47 nH and 433 MHz, values chosen to have nothing in common with it. One caveat the tests make explicit: feeding back the displayed ten-significant-digit reactance reproduces the input to about one part in ten trillion rather than exactly, because the displayed value is itself rounded. Feeding back the unrounded value is exact.

Why reactance rises with frequency, in one sentence: the faster the current changes, the larger the back-EMF the coil generates to oppose that change, so the same current amplitude requires a larger voltage — and a larger voltage for the same current is a larger impedance by definition.

A useful structural fact this page tests directly. X_L depends only on the *product* of frequency and inductance, so 100 mH at 1 kHz, 10 µH at 10 MHz and 100 nH at 1 GHz all give exactly the same 628.318530718 Ω. That identity is asserted across three decades of unit prefixes, because a unit-conversion bug would break it immediately while leaving a single worked example looking perfectly correct.

Phase and power. The positive imaginary sign in Z_L = +jX_L puts the voltage 90 degrees ahead of the current. OpenStax §15.4 gives average power as Eq. 15.12, P_ave = ½I₀V₀cos φ, and identifies cos φ = R/Z as the power factor; for a purely inductive element R is zero, φ is 90 degrees, and the average power is exactly zero. That is why the heat in a real inductor comes from its winding resistance and core loss, never from its reactance, and why a large reactive current in a power system costs conductor and transformer capacity without doing any work.

Rounding stage. Every quantity is carried at full decimal precision and rounded once, at the return boundary. Because this page spans twenty decades — nanohenries to henries, picohms to gigohms — the boundary rule is ten decimal places for values of one and above, and ten significant digits below one. A fixed ten-decimal-place rule would round the reactance of 1 nH at 1 mHz to exactly zero: a finite number, and therefore one that an ordinary output check would pass while being completely wrong.

What this does not model. Winding resistance, which sets the inductor's own Q and is the only thing that makes it dissipate. Core loss, which rises steeply with frequency and flux. Skin and proximity effect, which raise the effective winding resistance at high frequency. Inter-winding capacitance and the self-resonant frequency it creates, above which impedance falls with frequency. And core saturation, a current-dependent collapse of inductance that this page has no input for and therefore cannot warn about.

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.