Figure 1: An inductor’s reactance rises linearly with frequency. At DC (f=0) it acts as a short circuit. At high frequencies it acts as an open circuit.
Enter target Xₗ and one known value.
Table of Contents
What Is Inductive Reactance?
Inductive reactance (XL) is the opposition an inductor presents to alternating current. Unlike resistance, which dissipates energy as heat, reactance stores energy in a magnetic field during one quarter-cycle and returns it during the next. The effect is frequency-dependent: higher frequency means more opposition.
Inductive reactance is measured in ohms, just like resistance, but it causes the current to lag behind the voltage by 90°. This phase shift is what distinguishes reactance from resistance and is the reason we use complex numbers (impedance) to analyse AC circuits. The Impedance Calculator combines resistance and reactance into the full impedance picture.
The Formula
Where f = frequency (Hz), L = inductance (henries), ω = angular frequency (rad/s).
Impedance of a pure inductor: Z = jXL (purely imaginary, +90° phase)
The formula is linear in both frequency and inductance. Doubling the frequency doubles the reactance. Doubling the inductance also doubles the reactance. This makes XL straightforward to calculate and predict.
Worked Example — Motor Winding at 50 Hz
Step 1 — ω = 2π × 50 = 314.16 rad/s
Step 2 — XL = 314.16 × 0.05 = 15.71 Ω
This relatively low reactance at mains frequency means the motor draws significant magnetising current. At higher frequencies (such as in a variable-speed drive running at 100 Hz) the same winding would have XL = 31.4 Ω, drawing less current.
Worked Example — RF Inductor at 10 MHz
Step 1 — ω = 2π × 10×106 = 62.83×106 rad/s
Step 2 — XL = 62.83×106 × 1×10−6 = 62.83 Ω
Even a tiny 1 µH inductor presents substantial reactance at RF frequencies. This is why stray inductance from PCB traces and component leads becomes critical above a few MHz. The Resonant Frequency Calculator shows how this inductance interacts with parasitic capacitance to create unintended resonances.
Worked Example — Audio Crossover Inductor
Step 1 — XL = 2π × 3000 × 0.0015 = 28.27 Ω
In a speaker crossover, the inductor is placed in series with the woofer. At the crossover frequency, XL equals the speaker impedance (typically 8 Ω for a first-order crossover). Above the crossover frequency, XL increases and attenuates the signal, directing high frequencies to the tweeter instead.
How Frequency Affects XL
At DC (f = 0), XL = 0 — the inductor acts as a short circuit (just its wire resistance). As frequency increases, XL rises linearly. At very high frequencies the inductor acts almost as an open circuit. This behaviour is exploited in low-pass filters, where inductors in series block high frequencies while passing low frequencies.
On a log-log graph (as shown in the calculator above), XL vs frequency appears as a straight line with a slope of +1 decade/decade (+20 dB/decade). This is the opposite of capacitive reactance, which has a slope of −1.
Phase Relationship
In a pure inductor, the voltage leads the current by exactly 90°. The mnemonic “ELI the ICE man” helps remember this: in an E-L-I circuit, E (voltage) leads I (current). This 90° phase shift means the inductor alternately stores and returns energy without dissipating any — hence reactive power, not real power. The Phase Angle Calculator shows how this phase shift combines with resistance to produce the overall circuit phase angle.
XL vs XC — The Opposite Behaviours
Inductive reactance increases with frequency; capacitive reactance decreases with frequency. At the resonant frequency they are equal and cancel each other. Below resonance, XC > XL and the circuit is capacitive. Above resonance, XL > XC and the circuit is inductive. The Capacitive Reactance Calculator computes XC for the other half of this picture.
Frequently Asked Questions
Does an inductor have resistance too?
What happens at DC?
Can XL be negative?
What is the self-resonant frequency of an inductor?
How do I choose an inductor for a specific XL?
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