Capacitive Reactance Calculator

When alternating current became practical in the 1880s, engineers ran into a puzzle: a capacitor in an AC circuit drew current without dissipating heat, apparently in violation of Ohm's law. Heinrich Hertz and Oliver Heaviside, working independently in the late 1880s, showed that the current through a capacitor is 90° out of phase with the voltage — so the "resistance" felt by the source isn't ordinary resistance. Heaviside coined the term reactance around 1893 and developed the operational calculus that let engineers treat capacitors and inductors with the same algebra as resistors.

The formalism was completed by Charles Proteus Steinmetz at General Electric in 1893, when he published the use of complex numbers to represent AC impedance: Z = R + jX, where j = √−1 rotates the vector by 90°. Reactance became the imaginary component of impedance — and suddenly every AC circuit problem reduced to familiar DC-style algebra with complex arithmetic. That representation is still what every filter and power-factor-correction calculation uses today, 130 years later.

Physically, reactance is frequency-dependent opposition to AC. A capacitor dissipates zero average power because it returns stored energy to the source each cycle; only the resistive part of impedance turns electrical energy into heat. The frequency dependence — high X_c at low frequency, low X_c at high frequency — is the basis of every frequency-selective filter, every AC-coupling capacitor, and every power-supply bypass cap on your bench.

Pick what to solve for (Xc, frequency, or capacitance), enter the other two with standard suffixes (nF, µF, Hz, kHz, MHz), and this calculator returns the third using X_c = 1 / (2πfC). Preset buttons jump to common working points: 60 Hz line, 1 kHz audio, 1 MHz switching.

Everything runs client-side; no values leave your browser. For real capacitors at very high frequencies, ESR (equivalent series resistance) and parasitic inductance dominate beyond the self-resonant frequency — see the datasheet for your specific part. This calculator handles the ideal case.