About the Inductive Reactance Calculator
This inductive reactance calculator finds XL, the opposition an inductor presents to alternating current, from its inductance and the signal frequency. It also works backwards: give it a target reactance and either the frequency or the inductance, and it solves for the missing value. Units from nanohenries to henries and from hertz to gigahertz are supported, and an optional AC voltage gives the current that would flow through the inductor.
Use it when designing chokes and RF filters, picking an inductor for a crossover, estimating how much a coil impedes mains or audio frequencies, or checking textbook problems. The sweep chart shows how reactance climbs in a straight line with frequency for the inductance you chose.
The result applies to an ideal inductor. Real inductors also have winding resistance (DCR) and self-capacitance, so near the self-resonant frequency the actual impedance departs strongly from 2πfL; check the datasheet SRF for RF work.
How to use the inductive reactance calculator
- 1Choose whether to solve for reactance, inductance or frequency.
- 2Enter the known inductance and pick its unit (H, mH, µH or nH).
- 3Enter the known frequency and unit, or the target reactance in ohms.
- 4Optionally enter an AC voltage to see the current through the inductor.
- 5Read the result and use the chart to see XL across nearby frequencies.
Formula and method
An inductor resists changes in current by generating a back-EMF proportional to how fast the current changes. With a sinusoidal current the rate of change scales with frequency, so the opposition — the inductive reactance — is proportional to both the inductance and the frequency: XL = 2πfL, measured in ohms.
Rearranging gives the inductance needed for a target reactance at a known frequency, or the frequency at which a given inductor reaches that reactance. The current through the inductor is V ÷ XL and lags the voltage by 90°, so an ideal inductor stores and returns energy each cycle without dissipating any. At DC (f = 0) XL is zero and the inductor is a short circuit.
- XL
- Inductive reactance (Ω)
- f
- Frequency (Hz)
- L
- Inductance (H)
- ω
- Angular frequency 2πf (rad/s)
Worked examples
10 mH inductor at 1 kHz
XL = 2π × 1000 × 0.010 = 62.83 Ω. With 10 V RMS across it the current is 10 ÷ 62.83 ≈ 0.159 A, lagging the voltage by 90°.
Frequency where a 100 µH choke reaches 1 kΩ
f = XL ÷ (2πL) = 1000 ÷ (2π × 0.0001) ≈ 1.59 MHz. Above that frequency the choke presents more than 1 kΩ, which is why small chokes are used to block RF while passing DC.
Inductance for 377 Ω at 60 Hz
L = 377 ÷ (2π × 60) ≈ 1.000 H. That is the classic textbook result: ω at 60 Hz is 376.99 rad/s, so a 1 H inductor has about 377 Ω of reactance on mains frequency.
Frequently asked questions
What is inductive reactance?+
Inductive reactance (XL) is the opposition an inductor offers to alternating current, measured in ohms. Unlike resistance it does not dissipate power; it stores energy in the magnetic field and returns it each cycle.
What is the formula for inductive reactance?+
XL = 2πfL, where f is the frequency in hertz and L is the inductance in henries. For example a 1 mH inductor at 10 kHz has XL = 2π × 10,000 × 0.001 ≈ 62.8 Ω.
Why does inductive reactance increase with frequency?+
The voltage across an inductor equals L times the rate of change of current. Higher frequencies change current faster, so a given current needs more voltage — the inductor appears to have higher impedance.
How is inductive reactance different from capacitive reactance?+
Inductive reactance rises with frequency (XL = 2πfL) and makes current lag voltage, while capacitive reactance falls with frequency (XC = 1/2πfC) and makes current lead voltage. Where they are equal, an LC circuit resonates.
What is the reactance of an inductor at DC?+
At 0 Hz XL = 0, so an ideal inductor is a short circuit to steady DC. A real inductor only shows its winding resistance (DCR) once the current has settled.