Capacitor Reactance Calculator
Enter any two of frequency, capacitance, or capacitive reactance, the third is calculated instantly. Formula: XC = 1 ÷ (2πfC).
Last updated: May 2026
Enter any two values above to calculate the third.
XC = 1 ÷ (2πfC), enter f and C to get XC, or any other pair
Capacitive reactance, formula and use
Capacitive reactance XC is the opposition a capacitor presents to alternating current at a given frequency. Unlike resistance, it decreases as frequency rises, the opposite behaviour to inductive reactance. This inverse relationship makes capacitors effective for blocking DC, bypassing high-frequency noise, and setting the corner frequency of RC filters. The figure you get here feeds naturally into the impedance and inductor tools collected in the Electronics calculators hub, which cover the rest of an AC analysis.
Formula reference
| Solve for | Formula | Example |
|---|---|---|
| XC (Ω) | XC = 1 ÷ (2π × f × C) | f = 1 kHz, C = 100 nF → XC = 1592 Ω |
| f (Hz) | f = 1 ÷ (2π × XC × C) | XC = 1592 Ω, C = 100 nF → f = 1 kHz |
| C (F) | C = 1 ÷ (2π × f × XC) | XC = 1592 Ω, f = 1 kHz → C = 100 nF |
Typical use cases
- Finding the capacitor value needed for a target reactance at a given frequency
- Calculating the impedance of a coupling or bypass capacitor at the signal frequency
- Verifying that a decoupling capacitor presents low enough impedance at the noise frequency
- Designing RC or LC filter corners where capacitor reactance must equal a target resistance
Why XC falls while XL rises
Capacitive reactance is one piece of the broader AC analysis, and its inverse frequency behaviour is what sets it apart from the inductor. Here is where this calculator sits inside the AC sub-chain:
- Start with total impedance. The impedance calculator takes resistance, capacitive reactance and inductive reactance together and gives you Z, the number that governs current in any AC branch.
- You are here: capacitor reactance. XC = 1 / (2pfC) is the capacitor's contribution to that impedance. Because frequency sits in the denominator, XC falls as frequency rises: a capacitor blocks DC almost completely and becomes a near short-circuit at high frequencies. This is the built-in behaviour that makes capacitors useful for coupling, bypassing and filter corners.
- Compare against inductive reactance. An inductor does the opposite: XL = 2pfL, so its opposition rises with frequency. The inductor reactance calculator gives XL for the same analysis. When XC equals XL you have the resonant frequency of an LC circuit.
From the bench: the moment that clicked for me was putting a capacitor across a DC power rail with an AC ripple on it. At 50 Hz the capacitor has high reactance and barely filters anything; at 100 kHz it looks almost like a wire. That is why decoupling designs use small capacitors (low C, high XC at low f, low XC at high f) for high-frequency noise and larger ones for slow supply ripple.
Frequently Asked Questions
Why does capacitive reactance decrease at higher frequencies?
A capacitor stores charge. At high frequencies the voltage reverses quickly, so charge flows in and out continuously, the capacitor acts almost like a short circuit. At low frequencies and DC, the capacitor charges up and blocks further current, it acts like an open circuit. The formula XC = 1/(2πfC) captures this: frequency is in the denominator, so XC falls as f rises. This is the opposite of inductive reactance, which rises with frequency.
What is the difference between capacitive reactance and impedance?
Capacitive reactance XC = 1/(2πfC) describes only the frequency-dependent opposition from a pure capacitor. Impedance Z is the total opposition including any series resistance: Z = √(R² + XC²). For a real capacitor with equivalent series resistance (ESR), use the Impedance Calculator to find Z. At frequencies well below self-resonance, an ideal capacitor's impedance equals its reactance.
How do I choose a decoupling capacitor using reactance?
A decoupling capacitor should present an impedance much lower than the supply rail impedance at the noise frequency. Calculate XC at the noise frequency: enter the frequency and try different capacitor values until XC is well below the target (typically 1-10 Ω at the noise frequency). Common choices are 100 nF for MHz-range digital noise and 10-100 µF for lower-frequency supply ripple, combining both covers a wider frequency range.
How do I pick the right capacitor size for a bypass or coupling application?
Start from the frequency you care about, then target a reactance at least ten times lower than the surrounding impedance. For a 3.3 V microcontroller rail with switching noise around 10 MHz, a 100 nF ceramic gives a reactance near 0.16 Ohm, far below any practical rail impedance. For audio coupling at 20 Hz you usually want the reactance below 10 percent of the load impedance, which for a 10 kOhm load means at least 8 µF. Use the calculator to confirm the value before ordering.
Methodology and sources
This tool computes capacitive reactance, the frequency-dependent opposition a capacitor presents to alternating current, using the standard AC circuit relationship XC = 1 / (2πfC). Given any two of frequency, capacitance, or reactance, it rearranges the same formula to solve for the third.
- Method: XC = 1 / (2 × π × f × C), where f is frequency in hertz and C is capacitance in farads. Rearranged as f = 1 / (2 × π × XC × C) and C = 1 / (2 × π × f × XC). Worked example: f = 1 kHz, C = 100 nF gives XC = 1592 Ω.
- Standards and sources: Standard AC circuit physics, no proprietary standard is involved. The reactance of an ideal capacitor is a textbook result (e.g. Horowitz & Hill, The Art of Electronics), with all inputs reduced to SI base units (hertz, farads, ohms) before calculation.
- Assumptions and limits: Assumes an ideal capacitor with no equivalent series resistance (ESR) or inductance and operation well below the part's self-resonant frequency. It returns reactance only, not full impedance Z = √(R² + XC²); for a real capacitor with ESR, use the Impedance Calculator.
Reviewed and maintained by Rick Oosterling, who builds and wires 12 V, solar and EV systems hands-on. Last reviewed: June 2026. This calculator is a design and learning aid, not a substitute for a qualified professional or your local wiring and building code; verify any value before relying on it in a circuit that carries hazardous voltage or current.
Next step in this workflow
Reactance known: now calculate the full circuit impedance.