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.
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
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.
What units should I use for frequency and capacitance?
The formula uses SI base units: frequency in Hz and capacitance in farads (F). The unit selector handles kHz/MHz and µF/nF/pF automatically. Typical practical ranges: bypass and coupling capacitors are in the nF–µF range; RF capacitors and timing capacitors are often in the pF–nF range. Power supply bulk capacitance can reach hundreds of µF.
Next step in this workflow
Reactance known: now calculate the full circuit impedance.