Reactance Calculator

How to use

1. 1Pick a mode: inductive reactance, capacitive reactance, or resonance where the two are equal.
2. 2In a reactance mode, choose whether to solve for the reactance, the component value, or the frequency.
3. 3Enter the two known values and pick the right SI unit from each dropdown, for example µH, pF, or kHz.
4. 4Read the calculated reactance in ohms, or in resonance mode the resonant frequency and the matching reactance.
5. 5Copy the summary for your notes, or reset to return every field to its starting values.

About this tool

Reactance Calculator works out the opposition, measured in ohms, that an inductor or a capacitor presents to an alternating current at a chosen frequency, and the special frequency at which the two cancel. Reactance is different from resistance: a resistor opposes current by the same amount at every frequency and turns the energy into heat, while a reactance changes with frequency and stores energy in a magnetic or electric field and returns it each cycle, so an ideal inductor or capacitor consumes no real power and draws current 90 degrees out of phase with the voltage. The tool covers the three calculations engineers, students, and hobbyists actually search for. Inductive reactance follows X_L = 2 pi f L, so it grows in direct proportion to both the frequency and the inductance; a coil that is nearly a short circuit at mains frequency can present a large reactance in the radio bands, which is why inductors are used to block high frequencies and pass low ones. Capacitive reactance follows X_C = 1 / (2 pi f C), so it does the opposite and falls as frequency or capacitance rises; a capacitor is an open circuit at DC and an easier and easier path as frequency climbs, which is why capacitors couple signals while blocking direct current. Each of those two modes lets you fix any two of the three quantities, reactance, frequency, and the component value, and solves for the third, so the same tool answers what reactance a part has at a given frequency, what frequency produces a target reactance, or what component value you need to hit a reactance at a frequency. The third mode finds resonance. Because inductive reactance rises with frequency and capacitive reactance falls, there is exactly one frequency where they are equal; setting X_L = X_C and solving gives the resonant frequency f0 = 1 / (2 pi sqrt(L C)), and at that point both reactances equal the square root of L over C, the characteristic impedance of the pair. In a simple series or parallel LC circuit the two reactances cancel at resonance, which is the basis of tuned circuits, oscillators, RF matching, and loudspeaker crossovers. Values are entered with SI prefix dropdowns so a 100 nH coil, a 10 pF trimmer, or a 4.7 microfarad coupling cap are typed naturally, and every output is auto-scaled to a readable unit and reported alongside the angular frequency omega = 2 pi f. This is a reactance and resonance tool, not a filter corner calculator: it reports the ohms of opposition at the frequency you choose, which combine with any series resistance R to give the impedance magnitude, the square root of R squared plus X squared. All results are computed in your browser with plain trigonometry. The component and frequency values you enter are never uploaded, logged, or stored.

Free to use. Works in your browser. No signup, no login.