Capacitive Reactance Calculator — Xc = 1 / (2πfC)

Background.

Ask what resistance a 100 nF capacitor has and the answer depends on the signal. At 1 kHz it opposes current with about 1,592 ohms. At 100 kHz the same part offers about 16 ohms, and at DC it does not conduct at all. That frequency-dependent opposition is capacitive reactance, and it sits behind almost every job a capacitor does with a changing signal: passing audio from one stage to the next while blocking the DC bias, shunting high-frequency noise to ground beside a chip, setting the corner of a filter, tuning a radio circuit.

The relationship is Xc = 1/(2πfC). Doubling the frequency halves the reactance, and so does doubling the capacitance. You can run it three ways here: reactance from a frequency and a part value; the capacitance needed to reach a target reactance at a given frequency, which is how coupling and filter capacitors get sized; or the frequency at which a part you already have reaches a chosen reactance.

Reactance is measured in ohms, but it is not resistance. Voltage and current in an ideal capacitor are a quarter-cycle apart, with the current leading, so energy flows in during one part of the cycle and back out in the next and none of it becomes heat. A capacitor across a mains supply can carry a substantial current while consuming almost no real power; the slight warming of a real one comes from its small internal series resistance.

The equation describes an ideal component. Every real capacitor has some series inductance, and above its self-resonant frequency that inductance takes over, so the impedance stops falling and starts to climb.

What is capacitive reactance calculator?

Capacitive reactance, written Xc, is the ratio of the voltage amplitude across an ideal capacitor to the current amplitude through it when both are sine waves of one frequency. It is given in ohms as a positive magnitude. In complex notation the capacitor's impedance is −jXc, and that minus sign records that the current leads the voltage by 90°. The reciprocal, susceptance B = 1/Xc = ωC, is measured in siemens.

How to use this calculator.

1. 01Choose what to solve for: reactance, capacitance or frequency. Only the two inputs that mode needs are read, so a value left in the third field is ignored.
2. 02Enter each value with its unit. Use the marked capacitance, but bear in mind that class-2 ceramics such as X7R, X5R and Y5V can deliver much less under DC bias, which pushes the real reactance up; C0G/NP0 and film capacitors hold their value.
3. 03When solving for capacitance or frequency, enter the reactance as a positive number of ohms, kilohms or megohms; the 90° phase is understood.
4. 04To size a coupling capacitor, solve for capacitance at the lowest frequency that must pass, with a reactance of about a tenth of the load resistance.
5. 05For capacitors in parallel, add their susceptances, which is the same as adding their capacitances; for capacitors in series, add their reactances.

The formula.

A capacitor's current is proportional to how fast the voltage across it changes, i = C·dv/dt. For a sine wave at frequency f, taking that rate of change brings in a factor of ω = 2πf, the angular frequency in radians per second, and shifts the waveform by a quarter cycle. The ratio of voltage amplitude to current amplitude is therefore Xc = 1/(ωC) = 1/(2πfC), the form OpenStax University Physics gives, with f in hertz, C in farads and Xc in ohms.

Rearranged, the same equation gives C = 1/(2πf·Xc) and f = 1/(2πC·Xc). The three are exact inverses, so solving forwards and feeding the answer back returns the value you started with. Alongside the main answer you also get ω and the susceptance B = ωC.

With voltage and current 90° apart, the power factor cos φ is zero, and so is the average power ½V₀I₀ cos φ. The capacitor trades energy back and forth with the circuit rather than consuming it.

At 0 Hz the reactance would be infinite, so a frequency of zero is refused. The equation also leaves out what makes a real part non-ideal: series resistance, series inductance, leakage, and the drift of capacitance with voltage, temperature and age. Series inductance L sets the self-resonant frequency, 1/(2π√(LC)). Assuming 1 nH, a 100 nF part resonates near 15.9 MHz, and above that it behaves as an inductor.

A worked example.

Frequently asked questions.

References& sources.

1. [1]OpenStax (Rice University), University Physics Volume 2, §15.3 'RLC Series Circuits with AC'. Source of the defining equation used on this page: Eq. 15.3, X_C = 1/(ωC). Open access; retrieved 2026-07-29.
2. [2]James M. Fiore, AC Electrical Circuit Analysis: A Practical Approach, §8.2 'Series Resonance' (Engineering LibreTexts). Cited for the 2πf engineering convention used throughout this page and for the cancellation of capacitive against inductive reactance at resonance, Eq. 8.2 f₀ = 1/(2π√LC) and Eq. 8.13 Q = (1/R_T)√(L/C). Open access; retrieved 2026-07-29.
3. [3]OpenStax, University Physics Volume 2, §15.4 'Power in an AC Circuit'. Source of the average-power relation P_ave = ½I₀V₀cos φ (Eq. 15.12) and the identification of cos φ = R/Z as the power factor — together these are why an ideal capacitor, at φ = 90°, dissipates no average power. Open access; retrieved 2026-07-29.
4. [4]IEC 60050, International Electrotechnical Vocabulary (Electropedia), entries 131-12-48 'capacitive reactance' and 131-12-46 'reactance' (X = Im(Z)) — the normative source for the terminology. ACCESS NOTE: electropedia.org returns HTTP 403 to automated retrieval, so the entry numbers and terms were confirmed from the IEC's indexed entry titles but the definition text was not opened. No figure on this page is taken from it.