Capacitive Reactance Calculator
A capacitor does not have a fixed resistance to alternating current, instead it has capacitive reactance, written Xc, which falls as the frequency rises. At low frequencies a capacitor blocks the signal almost completely; at high frequencies it passes it freely. That single behaviour is what makes capacitors work as coupling caps, bypass caps, filters and timing elements. This calculator takes the frequency in hertz and the capacitance in microfarads and returns the capacitive reactance in ohms instantly, so you can size a coupling capacitor, design a high-pass corner, or check a power-supply bypass without doing the arithmetic by hand.
How to use the capacitive reactance calculator
-
1
Enter the frequency
Type the signal frequency in hertz (Hz). Mains power is 50 or 60 Hz; audio runs roughly 20 Hz to 20 kHz; RF is much higher.
-
2
Enter the capacitance
Type the capacitor value in microfarads (µF). Convert if needed: 1 nF = 0.001 µF and 1000 pF = 0.001 µF.
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3
Read the reactance
The capacitive reactance Xc appears immediately in ohms (Ω): no button to press, the result updates as you type.
The formula
Capacitive reactance is given by:
Xc = 1 / (2 × π × f × C)
where f is the frequency in hertz (Hz) and C is the capacitance in farads (F). The result Xc is in ohms (Ω). Reactance is inversely proportional to both frequency and capacitance: double the frequency and Xc halves; double the capacitance and Xc halves too. The angular form is Xc = 1 / (ω × C), where ω = 2 × π × f.
Because the input here is in microfarads, the tool first converts it: C (F) = C (µF) × 10⁻⁶.
Worked example
Take f = 60 Hz and C = 100 µF (= 100 × 10⁻⁶ F):
2 × π × f = 2 × π × 60 ≈ 376.99
Xc = 1 / (376.99 × 100 × 10⁻⁶)
= 1 / 0.037699
≈ 26.53 Ω
So a 100 µF capacitor presents about 26.5 Ω to a 60 Hz signal, small enough to be a useful bypass at mains frequency.
Reactance at common values
| Frequency (f) | Capacitance (C) | Reactance Xc |
|---|---|---|
| 60 Hz | 100 µF | ≈ 26.53 Ω |
| 60 Hz | 1 µF | ≈ 2 653 Ω |
| 1 kHz | 1 µF | ≈ 159.2 Ω |
| 10 kHz | 0.1 µF | ≈ 159.2 Ω |
| 1 MHz | 0.001 µF | ≈ 159.2 Ω |
Pitfalls to avoid
- Units matter. This tool expects hertz and microfarads. 1 nF = 0.001 µF and 1000 pF = 0.001 µF; 1 mF = 1000 µF. Mixing prefixes shifts Xc by orders of magnitude.
- Reactance is not resistance. Xc stores and returns energy rather than dissipating it, so it adds to resistance vectorially: total impedance is
Z = √(R² + Xc²), notR + Xc. - At DC (f → 0) the reactance is infinite. A capacitor blocks direct current; this calculator returns 0 for a zero frequency or zero capacitance as a safe guard, since the ideal value would be undefined.
- Real caps have ESR and leakage. The ideal Xc is an excellent estimate, but equivalent series resistance and a finite self-resonant frequency matter at high frequencies, so treat the figure as a starting point.
Frequently Asked Questions
Capacitive reactance (Xc) is the opposition a capacitor offers to alternating current, measured in ohms. It is given by Xc = 1 / (2π f C) and decreases as either the frequency or the capacitance increases, which is why capacitors pass high frequencies and block low ones.
Because Xc is inversely proportional to frequency. A higher frequency charges and discharges the capacitor more often each second, so more current flows for the same voltage, which means less opposition. At very high frequencies Xc approaches zero and the capacitor behaves almost like a short.
Convert to microfarads first: divide nanofarads by 1000 (1 nF = 0.001 µF) and divide picofarads by 1,000,000 (1000 pF = 0.001 µF). Then type that microfarad value into the capacitance field.
The values you enter are sent to our server only to compute the result. They are not stored or shared with anyone. In the step-by-step view, the values also travel in the page link between steps, so avoid sharing links that contain numbers you want to keep private.
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