What is the Capacitive & Inductive Reactance Calculator?
Reactance describes the imaginary part of impedance in a sinusoidal AC model. This calculator finds the magnitude for one ideal capacitor or inductor, or rearranges the same equation to find a component value or frequency. A magnitude is displayed as a positive number; the result also states the signed reactance and phase relationship.
It complements the Series & Parallel Capacitor Calculator, which combines capacitance values, and the RC Time Constant Calculator, which models DC transient response. A reactance value at one frequency is not a capacitor’s leakage resistance or an inductor’s winding resistance.
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Optional lcr meters (paid link) can measure components under the instrument’s stated conditions. Measurement frequency matters, and no purchase is required to use this calculator.
How to calculate capacitive or inductive reactance
- Choose Capacitor or Inductor.
- Choose the unknown: reactance magnitude, component value or frequency.
- Enter the two visible known quantities with their units.
- Use a positive AC frequency and a positive reactance magnitude, even for a capacitor.
- Calculate and review the magnitude, signed value, formula and steps.
Capacitance supports pF, nF, µF, mF and F; inductance supports nH, µH, mH and H. Frequency supports Hz, kHz, MHz and GHz; entered reactance supports Ω, kΩ and MΩ. Unit changes convert valid entries. Changing component type clears its value because capacitance and inductance are different quantities.
Optional inductor assortments (paid link) should be compared using their datasheets, including winding resistance and self-resonant frequency. Entered inductance is nominal; the tool does not estimate how it changes with current.
Capacitive and inductive reactance formulas
Inductor magnitude: |XL| = 2πfL
Capacitance: C = 1/(2πf|XC|)
Inductance: L = |XL|/(2πf)
Capacitor frequency: f = 1/(2πC|XC|)
Inductor frequency: f = |XL|/(2πL)
Use hertz, farads, henries and ohms. Angular frequency is ω = 2πf in radians per second. For an ideal capacitor, ZC = −j/(ωC) and signed XC is negative. For an ideal inductor, ZL = jωL and signed XL is positive. The symbol j denotes the imaginary unit.
Doubling frequency halves capacitive reactance and doubles inductive reactance. This frequency dependence is why the same component has different reactance at different signal frequencies. A single ideal component has no real resistance in this model.
Reactance calculation examples at 50 Hz and 1 kHz
Capacitive example: C = 100 µF at f = 50 Hz. Convert C to 0.0001 F. Then |XC| = 1/(2π × 50 × 0.0001) ≈ 31.83099 Ω. Signed XC is about −31.83099 Ω, and ideal capacitor current leads voltage by 90°. At 100 Hz, the magnitude halves to about 15.91549 Ω.
Inductive example: L = 10 mH = 0.01 H at f = 1 kHz = 1,000 Hz. |XL| = 2π × 1,000 × 0.01 ≈ 62.83185 Ω. Signed XL is positive, and ideal inductor current lags voltage by 90°.
Inverse example: an ideal capacitor with a 100 Ω reactance magnitude at 1 kHz needs C = 1/(2π × 1,000 × 100) ≈ 1.591549 µF. The inverse modes add a fifth step that checks the calculated value with the forward reactance formula.
Understand magnitude, sign and phase
The main reactance result is |X|, a nonnegative magnitude in ohms. The sign describes energy-storage behavior: capacitors have negative signed reactance and inductors positive signed reactance. It does not mean a negative physical resistance or negative component value.
A 90° phase relationship applies to one ideal component in sinusoidal steady state. It does not describe the total phase of a circuit containing resistance and multiple reactive elements. Current and voltage amplitude are not inputs, so this calculator does not report current, power, component stress or a permitted operating voltage.
Optional capacitor assortments (paid link) must be checked for tolerance, dielectric behavior, ESR and ratings. Use the Ohm’s Law Calculator for a separate resistive calculation; a reactance magnitude should not be silently treated as a resistor for real-power calculations.
Accuracy and limitations of ideal AC reactance
Entered frequency must represent 1e−9–1e12 Hz; capacitance 1e−15–1,000 F; inductance 1e−12–1,000,000 H; and reactance magnitude 1e−12–1e18 Ω. All active inputs must be positive and finite. DC at 0 Hz is excluded: ideal capacitor reactance tends to infinity, while ideal inductor reactance tends to zero.
These wide computational ranges are not guaranteed operating ranges of real parts. Calculated inverse results may fall outside the allowed entry ranges. The tool excludes ESR, winding resistance, core losses, saturation, parasitic capacitance/inductance, self-resonance, tolerance and frequency-dependent component values. Near or above self-resonance, a real part can behave differently from its ideal label.
Optional digital oscilloscopes (paid link) can support appropriate waveform measurements. This page is not a combined RLC impedance, resonance, mains wiring or component-rating tool. Use the 555 Timer Calculator for a separate standard timer frequency estimate, without assuming that its waveform is sinusoidal.
Capacitive & Inductive Reactance Calculator FAQs
The magnitude is |XC| = 1/(2πfC), with frequency in hertz and capacitance in farads. Signed capacitive reactance is negative.
The magnitude is |XL| = 2πfL, with frequency in hertz and inductance in henries. Signed inductive reactance is positive.
Yes. Choose component value, select capacitor or inductor, then enter positive reactance magnitude and frequency. The result is checked with the forward formula.
The input is a magnitude. The calculator applies the capacitive negative sign separately and explains the ideal phase relationship.
No. This tool requires positive AC frequency. In the ideal DC limit, capacitor reactance tends to infinity and inductor reactance tends to zero.
No. It models one ideal capacitor or inductor at one sinusoidal frequency. Combined circuit impedance, real losses and self-resonance are outside scope.
Sources and methodology
Independently implemented standard ideal-circuit equations, reviewed October 26, 2026. Linked organizations do not endorse this tool. The exact manufacturer datasheet governs real component limits.