Inductive Reactance Calculator

Introduction to inductive reactance in AC coils

Inductive reactance is the part of an inductor’s opposition to alternating current that grows with both frequency and inductance. The formula is short, but useful results depend on entering values in the correct base units and understanding which components belong to the same circuit. This calculator turns frequency and inductance into X_L. When resistance and capacitance are supplied, it also estimates X_C, series impedance, phase angle, and resonant frequency for that test case.

A coil does not have one fixed AC opposition. At a higher frequency, the same inductance has a larger X_L; at DC, its ideal inductive reactance is zero. Adding a capacitor changes the picture because capacitive reactance moves in the opposite direction with frequency. The result panel helps distinguish a pure-inductor question from a simple series RLC estimate, so the number is easier to connect to a real tuning, filtering, or driver-stage decision.

Use the calculator as a transparent first pass: enter the electrical quantities, check the direction of the result, and then compare the estimate with component data or bench measurements when the design matters.

What AC coil question does this calculator answer?

This inductive reactance calculator answers how much a coil opposes sinusoidal AC at a chosen frequency. It can also answer related questions: whether an inductor and capacitor pair are near resonance, whether the overall series branch is net inductive or net capacitive, and how much winding resistance changes the magnitude and phase of impedance.

Start by phrasing the task plainly. You might ask, “What is the reactance of this 10 mH choke at 1 kHz?” or “At what frequency will this inductor and capacitor resonate?” If the circuit includes a capacitor, ensure it is in the same series model represented by the calculation. A capacitor in a parallel branch or an inductor with substantial parasitics requires a different circuit model.

How to use the inductive reactance calculator

Enter the drive frequency in hertz and the inductance in henries, then select Compute Reactance. Frequency and inductance must be greater than zero. Series resistance and capacitance are optional: leave them blank or at zero when you only need X_L.

1. Enter the AC frequency f in Hz. Convert kHz or MHz first; for example, 1 kHz becomes 1000 Hz.
2. Enter inductance L in H. Convert mH and µH first; 10 mH is 0.01 H and 100 µH is 0.0001 H.
3. Enter winding or added series resistance R in Ω when you want impedance and phase.
4. Enter capacitance C in F when you want capacitive reactance and the ideal LC resonant frequency. One microfarad is 0.000001 F.
5. Read X_L first, then use the additional RLC outputs to judge balance, phase, and proximity to resonance.

When comparing scenarios, change one value at a time where possible. That makes the expected trend clear: frequency raises X_L, while it lowers X_C.

Inductance, frequency, resistance, and capacitance inputs

The form uses base SI units so that its calculations remain unambiguous. Datasheets often list coils in mH or µH and capacitors in nF or µF, so conversion is the most important input check. Entering “10” for a 10 mH coil would model 10 H, not 0.01 H, and make the reported reactance 1000 times too large.

Frequency should be the frequency of the sinusoidal signal being evaluated, not a bandwidth or switching-edge rate. Resistance represents a series loss term, commonly the coil’s DC winding resistance for a simple estimate. At high frequency, equivalent series resistance and core losses can differ from that DC value. Capacitance is used here as the capacitor in an idealized series RLC relationship; it is not automatically the coil’s distributed self-capacitance.

Defaults are merely convenient starting values. For a useful design comparison, use measured values or a datasheet value at a relevant test frequency, then try plausible tolerance limits. That range is often more informative than a single overly precise estimate.

Formulas for X_L, X_C, impedance, and resonance

The inductive reactance calculation uses the standard sinusoidal AC relationship. Frequency is in hertz, inductance is in henries, and the resulting reactance is in ohms.

For an entered capacitor, the calculator also evaluates capacitive reactance and the ideal LC resonant frequency:

For the simple series RLC model, the net reactive component is X_L − X_C. The displayed magnitude is √(R² + (X_L − X_C)²), and phase is atan2(X_L − X_C, R) expressed in degrees. A positive phase is net inductive; a negative phase is net capacitive. At ideal resonance, X_L and X_C balance, leaving series resistance as the main impedance term.

Worked example: a 10 mH coil at 1 kHz

Consider a 10 mH coil at 1 kHz with 8 Ω of series resistance and a 1 µF series capacitor. Enter f = 1000 Hz, L = 0.01 H, R = 8 Ω, and C = 0.000001 F. The calculator reports X_L = 62.83 Ω and X_C = 159.15 Ω. The ideal resonant frequency is approximately 1591.55 Hz.

At 1 kHz, the capacitive term is larger than the inductive term, so the branch is net capacitive. The impedance magnitude is about 96.66 Ω and the phase angle is about −85.25°. Raising the frequency moves the two reactances toward each other because X_L rises while X_C falls. Near 1591.55 Hz, the reactive terms cancel in this ideal series model and the impedance approaches the 8 Ω resistance value.

Frequency sensitivity for the same RLC test case

The same 10 mH, 8 Ω, 1 µF test case shows why frequency needs to be specified whenever reactance is discussed.

This direction check is a useful way to spot mistakes. If frequency rises and a reported X_L falls, revisit the entered units or the assumed circuit topology.

How to interpret inductive reactance results

For a coil by itself, X_L is the primary answer. Compare it with source, load, or winding resistance to decide whether the coil behaves mainly as a reactive element at that frequency. A much larger X_L than resistance means the branch is predominantly inductive; a similar-sized resistance means losses have a more visible effect.

With a capacitor entered, use the sign of phase and the relative sizes of X_L and X_C. Below ideal resonance, a series LC pair is normally net capacitive; above it, it is normally net inductive. The impedance magnitude alone does not state which side of resonance you occupy, which is why the phase readout is included.

Limitations and assumptions for real coils

This inductive reactance calculator uses ideal linear AC relationships and a simple series RLC interpretation. Real coils can depart from that model because of core saturation, temperature, skin effect, distributed winding capacitance, core loss, and frequency-dependent resistance. At sufficiently high frequency, a coil can approach self-resonance and no longer behave like an ideal inductor.

The resonance result is therefore an ideal LC estimate, not a guarantee of a finished circuit’s exact peak or notch frequency. Lead inductance, capacitor tolerance and ESR, layout, loading, and whether components are actually in series or parallel can shift the observed result. Treat this page as a fast calculation and sanity-check tool, then verify critical designs with component models and measurement equipment.