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

Background.

Capacitive reactance is the opposition a capacitor presents to alternating current, measured in ohms like resistance but behaving nothing like it. It falls as frequency rises: the same capacitor that blocks a DC bias almost completely can look like a fraction of an ohm to a radio-frequency signal. That single inverse relationship is why capacitors are used to couple signals while blocking DC, why decoupling capacitors work at all, and why a capacitor that is perfect at one frequency is useless at another.

The equation is Xc = 1 divided by 2πfC, with frequency in hertz and capacitance in farads. This calculator runs it in all three directions, because in practice you are as likely to be solving backwards as forwards. Choose reactance and you get ohms from a frequency and a part value. Choose capacitance and you get the part you need to hit a target impedance at a given frequency — the calculation behind sizing a coupling capacitor so that its reactance is small compared with the load it feeds. Choose frequency and you find where a capacitor you already have reaches a particular reactance, which is the calculation behind a crossover point or a decoupling corner.

One convention, stated plainly because it causes more confusion than the arithmetic does: reactance is shown here as a positive magnitude. In complex notation a capacitor's impedance is minus j times Xc, which means the current through it leads the voltage across it by exactly 90 degrees, and because the phase angle is 90 degrees an ideal capacitor dissipates no average power at all — it stores energy and returns it every cycle. Some texts write capacitive reactance as a negative number to carry that sign information. This page keeps the magnitude positive and states the phase separately, which is what almost every datasheet and bench measurement does.

The susceptance output is the reciprocal, B equals 1 over Xc, in siemens. It is worth having because susceptances add directly when capacitors are in parallel, in the same way conductances do, whereas reactances do not. For a parallel bank the susceptance form is simply less error-prone.

The scope limit belongs next to the answer rather than in a footnote. This equation describes an ideal capacitor: no equivalent series resistance, no equivalent series inductance, no leakage, no dielectric loss. Every real capacitor has lead and plate inductance, and above its self-resonant frequency that inductance dominates, so the impedance stops falling with frequency and starts rising. A 100 nF ceramic in a small surface-mount package typically turns that corner somewhere in the tens of megahertz, and above it the part is behaving as an inductor no matter what the marking says. That is why decoupling networks use several values in parallel rather than one large one, and it is the single most important caveat on this page.

One more practical point about real parts. Class-2 ceramic dielectrics such as X7R and Y5V lose a substantial fraction of their marked capacitance under DC bias and continue to lose more as they age. A 10 µF X5R rated at 6.3 V can deliver less than half its marked value at 5 V of bias, which doubles the real reactance. Where the reactance genuinely matters, C0G/NP0 or film parts hold their value; where it does not, the derating is usually the difference between a calculation and a measurement.

What is capacitive reactance calculator?

Capacitive reactance, Xc, is the magnitude of the opposition an ideal capacitor offers to a sinusoidal current at a given frequency. It has the unit of the ohm, and like resistance it relates the voltage across the component to the current through it, but the resemblance stops there. Reactance stores energy rather than dissipating it, and because the voltage and current are 90 degrees out of phase the average power in a purely reactive element is zero. Its value is Xc = 1/(ωC) = 1/(2πfC), so it is inversely proportional to both frequency and capacitance: doubling either one halves the reactance. At DC, where f is zero, the reactance is infinite, which is the formal statement of the familiar fact that a capacitor blocks DC. In complex impedance notation the capacitor contributes −j/(ωC), and the negative imaginary sign is what encodes the phase: current leads voltage. The reciprocal quantity, susceptance B = 1/Xc = ωC, is measured in siemens and is the form that adds directly across parallel elements. Reactance is the property behind every capacitor application that involves a changing signal — coupling, decoupling, filtering, tuning, timing and snubbing — and it is why the same physical part behaves completely differently at 50 Hz and at 500 MHz.

How to use this calculator.

1. 01Pick what you want to solve for. Reactance is the forward direction; capacitance and frequency are the two inverses, and all three use the same equation rearranged.
2. 02Enter the two known quantities with their units. The third field is ignored in that mode, so a leftover value there will not be rejected.
3. 03Read the reactance in ohms. Compare it against the impedance it works alongside — a coupling capacitor is usually sized so that its reactance at the lowest frequency of interest is a tenth or less of the load resistance.
4. 04Use the capacitance mode to size a part: enter the frequency and the reactance you want, and the calculator returns the farads needed.
5. 05Use the frequency mode to find where a part you already have hits a target reactance — the corner of a decoupling network or a crossover point.
6. 06Read the susceptance if you are working with capacitors in parallel; susceptances add directly, reactances do not.
7. 07Check the self-resonance caveat before trusting the answer at high frequency. Above a real capacitor's self-resonant frequency its impedance rises rather than falls, and this equation no longer describes it.

The formula.

The equation. A capacitor's current is C times the rate of change of its voltage. For a sinusoid at angular frequency ω that derivative introduces a factor of ω, so the ratio of voltage amplitude to current amplitude is 1/(ωC) — the reactance. OpenStax's University Physics Volume 2 prints it at §15.3 as Eq. 15.3, X_C = 1/(ωC). Substituting ω = 2πf gives the engineering form used everywhere on this page, X_C = 1/(2πfC).

The three modes are the same equation rearranged, and they are exact inverses of one another. Solve for reactance and feed the answer back in capacitance mode with the same frequency, and the original capacitance comes back to full precision; do the same in frequency mode and the original frequency comes back. That round trip is asserted as a test, at the worked example and again at 22 pF and 47 MHz, values chosen to have nothing in common with it.

Phase and power. The negative imaginary sign in Z_C = −jX_C puts the current 90 degrees ahead of the voltage. OpenStax §15.4 gives average power as Eq. 15.12, P_ave = ½I₀V₀cos φ, and identifies cos φ = R/Z as the power factor; for a purely capacitive element R is zero, φ is 90 degrees, and the average power is exactly zero. That is not an approximation — it is why a capacitor can carry large circulating currents without heating, and why the only heat in a real capacitor comes from its equivalent series resistance rather than from its reactance.

Why the reactance falls with frequency. The capacitor's job each half cycle is to accept a charge equal to C times the voltage swing. At higher frequency the same charge has to move in less time, so the current is larger for the same voltage, and a larger current for the same voltage is by definition a lower impedance. The inverse proportionality is exact, and the calculator asserts it: doubling f halves X_C, and a factor of ten in f gives a factor of ten in X_C, both to the last digit computed.

Rounding stage. Every quantity is carried at full decimal precision and rounded once, at the return boundary. Because this page spans twenty decades of magnitude — picofarads to farads, nano-ohms to gigohms — the rounding rule is ten decimal places for values of one and above, and ten significant digits below one. A fixed ten-decimal-place rule would round 22 picofarads to exactly zero, which is a finite number and would therefore pass an ordinary output check while being completely wrong.

What this does not model. Equivalent series resistance, which is what actually heats a capacitor and what dominates an electrolytic's impedance above about 100 kHz. Equivalent series inductance, which sets the self-resonant frequency above which the impedance rises with frequency instead of falling. Leakage and dielectric absorption. Voltage and temperature coefficients — a class-2 ceramic can lose more than half its marked capacitance under DC bias, doubling the reactance this page computes. And the distinction between a two-terminal part and a mounted one: the loop inductance of the pads and vias often exceeds the capacitor's own.

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.