Capacitive Reactance Calculator
The reactance a capacitor presents at a given frequency, the current it passes at a voltage, and — with an inductor entered — the inductive reactance and the resonant frequency of the pair.
A capacitor opposes alternating current with a reactance X_C = 1 ÷ (2πfC) that falls as frequency rises; an inductor’s X_L = 2πfL rises with it.
How the capacitive reactance calculator works
A capacitor opposes alternating current with a reactance X_C = 1 ÷ (2πfC) that falls as frequency rises; an inductor’s X_L = 2πfL rises with it. Where the two are equal the circuit resonates, at f₀ = 1 ÷ (2π√(LC)). Reactance is in ohms and sets the current just as resistance does, but with a 90° phase shift and no power dissipated.
Formula: X_C = 1 ÷ (2π f C); X_L = 2π f L; f₀ = 1 ÷ (2π √(L C))
Worked examples
| Inputs | Capacitive reactance (Ω) | Note |
|---|---|---|
| 10 µF at 50 Hz | 318.31 | 318.3 Ω |
| 100 nF at 1 kHz | 1,591.55 | 1,591.5 Ω |
| 10 µF with 10 mH | 318.31 | resonates at 503 Hz |
FAQFrequently asked questions
Why does a capacitor’s reactance fall with frequency?
Current through a capacitor is the rate of change of charge, and a faster-changing voltage moves charge faster; more current for the same voltage means less opposition. At DC (zero frequency) the reactance is infinite — the capacitor blocks it.
Is reactance the same as resistance?
Both are in ohms and both set the current, but reactance shifts the current 90° out of phase with the voltage and dissipates no power. In a circuit with both, they add as vectors to give impedance.
What is resonance used for?
At the resonant frequency the reactances cancel, so a series LC circuit passes that frequency freely and a parallel one blocks it — the basis of radio tuning, filters and oscillators.
What is the ten-times-frequency row for?
To show the slope: reactance is inversely proportional to frequency, so ten times the frequency gives a tenth of the reactance. It is why a small capacitor can pass high frequencies while blocking mains hum.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Physical Laboratory — the UK's national measurement institute
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.