Inductive Reactance Calculator
Calculate the inductive reactance (XL) of an inductor at a given frequency. Inductive reactance is the opposition to AC current flow caused by an inductor, measured in ohms. See also our Capacitive Reactance Calculator and Resonant Frequency Calculator.
How to Calculate Inductive Reactance
Inductive reactance (XL) is the opposition that an inductor presents to alternating current. Unlike resistance, which dissipates energy as heat, reactance stores energy in a magnetic field and returns it to the circuit each half cycle. Inductive reactance increases linearly with both frequency and inductance — higher frequencies or larger inductors create more opposition to current flow.
In an ideal inductor, current lags voltage by exactly 90 degrees. This phase relationship is fundamental to understanding AC circuits, power factor, and filter behavior. Real inductors also have DC resistance (DCR) and parasitic capacitance, which affect their behavior at high frequencies.
Inductive Reactance Formula
Inductive Reactance:
XL = 2πfL = ωL
Where:
XL = inductive reactance (Ω)
f = frequency (Hz)
L = inductance (H)
ω = angular frequency = 2πf (rad/s)
Impedance of Inductor:
Z = jXL = j2πfL (complex notation)
|Z| = XL (magnitude)
Current Through Inductor:
I = V / XL (magnitude)
Phase: I lags V by 90°
With Series Resistance:
|Z| = √(R² + XL²)
θ = arctan(XL/R)
Example Calculation
Calculate the inductive reactance of a 10mH inductor at 1kHz:
L = 10 mH = 0.01 H
f = 1 kHz = 1000 Hz
XL = 2π × 1000 × 0.01 = 62.83 Ω
If 10V AC is applied:
I = V/XL = 10/62.83 = 0.159 A = 159 mA
Current lags voltage by 90°
At 10 kHz (10× frequency):
XL = 2π × 10000 × 0.01 = 628.3 Ω (10× reactance)
I = 10/628.3 = 15.9 mA (1/10 the current)
This demonstrates why inductors block high frequencies
and pass low frequencies (low-pass filter behavior)
Inductive Reactance Reference Table
| Inductance | Frequency | XL (Ω) |
|---|---|---|
| 1 µH | 1 MHz | 6.28 Ω |
| 10 µH | 1 MHz | 62.83 Ω |
| 100 µH | 1 MHz | 628.3 Ω |
| 100 µH | 100 kHz | 62.83 Ω |
| 1 mH | 1 kHz | 6.28 Ω |
| 1 mH | 10 kHz | 62.83 Ω |
| 10 mH | 1 kHz | 62.83 Ω |
| 10 mH | 50 Hz | 3.14 Ω |
| 10 mH | 60 Hz | 3.77 Ω |
| 100 mH | 50 Hz | 31.42 Ω |
| 100 mH | 60 Hz | 37.70 Ω |
| 1 H | 60 Hz | 377.0 Ω |
Frequently Asked Questions
What is the difference between reactance and resistance?
Resistance (R) dissipates energy as heat and is independent of frequency. Reactance (X) stores and returns energy without dissipation and varies with frequency. Inductive reactance increases with frequency (XL = 2πfL), while capacitive reactance decreases (XC = 1/(2πfC)). Together, resistance and reactance form impedance: Z = R + jX.
Why does current lag voltage in an inductor?
An inductor opposes changes in current (Lenz's Law). When voltage is applied, the inductor's back-EMF initially prevents current from flowing. Current builds up gradually as the magnetic field establishes. The result is that current reaches its peak 90° (quarter cycle) after voltage reaches its peak — current lags voltage.
What is the self-resonant frequency of an inductor?
Every real inductor has parasitic capacitance between its windings. This creates a self-resonant frequency (SRF) above which the inductor behaves as a capacitor. The inductor is only useful below its SRF. For example, a 10mH inductor might have SRF of 500kHz. Above this frequency, its impedance decreases rather than increases. Always check the SRF specification when selecting inductors for high-frequency applications.
How does inductive reactance affect power factor?
Inductive reactance causes current to lag voltage, creating a lagging power factor. The power factor equals cos(θ) where θ = arctan(XL/R). Pure inductance has PF = 0 (all reactive power, no real power). Adding resistance improves PF toward 1.0. Capacitors are used to cancel inductive reactance and correct power factor in industrial installations.
What is the reactance of a transformer winding?
Transformer windings have both leakage reactance (due to flux that doesn't couple between windings) and magnetizing reactance (due to the core's magnetizing inductance). Leakage reactance limits short-circuit current and causes voltage regulation issues. Magnetizing reactance determines no-load current. Both are frequency-dependent and specified at the rated frequency (50/60 Hz).
How do I measure inductance if I only know reactance?
Rearrange the formula: L = XL/(2πf). Measure the voltage across and current through the inductor at a known frequency, calculate XL = V/I, then solve for L. For example, if an inductor passes 100mA at 10V and 1kHz: XL = 10/0.1 = 100Ω, L = 100/(2π×1000) = 15.9mH. Ensure the measurement frequency is well below the self-resonant frequency.
Inductors as Frequency-Dependent Components
The frequency dependence of inductive reactance makes inductors essential for frequency-selective circuits. At DC (f=0), an ideal inductor has zero reactance — it acts as a short circuit (just wire resistance). As frequency increases, reactance increases linearly, progressively blocking higher frequencies. This property is exploited in low-pass filters (series inductor blocks high frequencies), high-pass filters (shunt inductor passes low frequencies to ground), and chokes (block AC while passing DC in power supplies).
Practical Applications
- EMI filters: Ferrite beads and chokes block high-frequency noise on power and signal lines
- Crossover networks: Audio crossovers use inductors to direct low frequencies to woofers
- Power supplies: Output inductors in switching regulators smooth current ripple
- Motor drives: Line reactors limit current harmonics from VFDs
- RF circuits: Inductors set impedance and frequency response in RF amplifiers
- Current limiting: Series reactors limit fault current in power distribution