Inductive Reactance Calculator
Calculates inductive reactance (XL) for a coil or inductor in an AC circuit at a given frequency. Used in RF design, transformer analysis, filter circuit development, and motor analysis.
Last updated: May 2026
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About this calculator
Inductive reactance (XL) is the opposition an inductor presents to alternating current, caused by the back-EMF (Lenz's law) generated as current changes. The formula is XL = 2π × f × L, where f is the frequency in hertz (Hz) and L is the inductance in henries (H). Since inductance is commonly specified in millihenries (mH), the calculator converts it: L(H) = L(mH) / 1,000, giving XL = 2 × π × f × (L_mH / 1,000). Variables: frequency (Hz), inductance (mH). Reactance is expressed in ohms (Ω). Unlike capacitive reactance, inductive reactance increases with frequency — an inductor passes low-frequency currents easily and increasingly opposes high-frequency signals; at DC (f = 0), XL = 0 and the inductor acts like a plain wire. Reference values at 60 Hz: 1 mH → 0.377 Ω; 100 mH → 37.7 Ω; 1 H → 377 Ω. Edge cases: the formula assumes an ideal inductor with no winding resistance (DCR), no core losses, and no parasitic capacitance. Real inductors have DCR (causes I²R losses), core losses (hysteresis and eddy currents in ferromagnetic cores), and parasitic shunt capacitance that creates a self-resonant frequency (SRF). Above the SRF, the inductor behaves like a capacitor — the formula gives wrong results. Core saturation at high currents reduces effective inductance dramatically; ferrite cores especially must be operated below their saturation flux density. For full AC analysis, complex impedance Z_L = jωL = +jXL captures the 90° phase lead of voltage over current. Pure inductors store energy in their magnetic field and dissipate none (in the ideal case), but real inductors always have some DCR and core loss.
How to use
Example 1: 50 mH inductor at 1 kHz (audio frequency). Step 1: convert inductance — L = 50 / 1,000 = 0.05 H. Step 2: XL = 2π × 1000 × 0.05 = 6.283 × 1000 × 0.05 ≈ 314.16 Ω. Verify: at 60 Hz (US mains), the same inductor has XL = 2π × 60 × 0.05 ≈ 18.85 Ω — proportionally smaller at lower frequency, confirming linear relationship. Example 2: 1 mH RF choke at 100 kHz. Step 1: L = 1 / 1,000 = 0.001 H. Step 2: XL = 2π × 100000 × 0.001 = 6.283 × 100000 × 0.001 ≈ 628.3 Ω. Verify: at 1 MHz (10× higher), XL would be 6283 Ω — 10× larger as frequency rises 10×, consistent with linear f dependence.
Frequently asked questions
What is inductive reactance and how is it different from resistance?
Inductive reactance (XL) is the frequency-dependent opposition an inductor offers to alternating current, measured in ohms. Resistance dissipates energy as heat (P = I²R) and is constant regardless of frequency. Inductive reactance, by contrast, stores energy in a magnetic field and releases it back to the circuit each cycle without dissipation (in the ideal case), and its magnitude increases with frequency. Both affect current flow, but only resistance causes real power loss; inductive reactance causes a 90° phase shift between voltage and current — voltage leads current in an inductor. Combined R and XL in series form an RL circuit with impedance Z = √(R² + XL²) and phase angle φ = arctan(XL/R).
How does frequency affect inductive reactance?
Inductive reactance is directly proportional to frequency: XL = 2πfL. Doubling the frequency doubles the reactance. At DC (0 Hz), XL is zero and the inductor acts like a plain wire (only its DCR remains). At very high frequencies, XL becomes large enough to effectively block AC signals — this is why inductors are used as chokes in power supplies (passing DC while blocking high-frequency noise) and as low-pass filter elements in audio and RF circuits. The same property makes RFI/EMI suppression chokes effective at killing high-frequency interference. However, above the inductor's self-resonant frequency (SRF), parasitic shunt capacitance dominates and the component starts behaving like a capacitor — XL no longer applies.
How do I calculate total impedance in a circuit with both inductive reactance and resistance?
When a resistor and inductor are in series, the total impedance Z is not simply R + XL because resistance and reactance are 90° out of phase (voltage across the inductor leads current by 90°, voltage across the resistor is in phase). Instead, use the Pythagorean formula: Z = √(R² + XL²). For example, a 300 Ω resistor in series with an inductor showing 400 Ω of reactance gives Z = √(300² + 400²) = √(90,000 + 160,000) = √250,000 = 500 Ω. The phase angle is φ = arctan(XL/R) = arctan(400/300) ≈ 53.1°. This impedance value governs the current drawn from the source via Ohm's Law: I = V / Z. For parallel RL combinations, use admittance: Y = 1/R + 1/(jXL), then Z = 1/Y. RLC circuits combine all three with both XL and Xc.
What are common mistakes when computing inductive reactance?
Forgetting to convert inductance from mH to H (divide by 1,000) gives XL that is 1,000× too small. Confusing inductive (XL = 2πfL, increases with frequency) with capacitive reactance (Xc = 1/(2πfC), decreases with frequency). Treating XL as the only impedance and ignoring the inductor's DCR (winding resistance) at low frequencies — DCR is usually significant when XL is small. Ignoring core saturation at high currents — once a ferromagnetic core saturates, effective L drops dramatically and XL is no longer well-defined. Using the formula above the self-resonant frequency where parasitic capacitance dominates. Forgetting that XL has a +90° phase angle when combining with R in impedance calculations. Ignoring the inductor's tolerance (±5% to ±20% is typical) when designing precision filters. Mixing up inductance units — μH and mH and H — by factors of 1,000.
When should I NOT rely on the basic XL formula?
Above the inductor's self-resonant frequency (SRF, typically 1 MHz to 1 GHz depending on construction), parasitic shunt capacitance dominates and the device behaves like a capacitor — XL no longer applies. At high current, ferromagnetic cores can saturate (B reaches Bsat), causing effective inductance to collapse and XL to drop sharply. Air-core inductors avoid saturation but are physically large for a given inductance. At high frequencies, skin effect and proximity effect in the winding increase AC resistance well above DCR. For high-power applications, core losses (hysteresis loss ∝ f, eddy-current loss ∝ f²) become significant and the inductor heats up; manufacturer datasheets provide derating curves. Transformers and coupled inductors require mutual inductance analysis, not just XL of one winding. Variable inductors (saturable reactors, fluxgate cores) have nonlinear behavior. For switching converters, the inductor is intentionally operated near saturation in some designs — use SPICE or specialized power-electronics calculators rather than this formula.