What this calculator does
Capacitive reactance is the opposition a capacitor presents to alternating current, measured in ohms just like resistance, but with one key difference: it depends on frequency. A capacitor blocks direct current almost completely once charged, but at higher AC frequencies it lets more current through, so its reactance falls as frequency rises.
This behaviour is the opposite of an inductor, whose reactance rises with frequency. That contrast is why capacitors and inductors are used together in filters: a capacitor can be chosen to pass high frequencies while blocking low ones, or vice versa, depending on how the circuit is arranged.
The formula
Capacitive reactance formula: Xc = 1 ÷ (2πfC), where f is the frequency in hertz and C is the capacitance in farads. Since real capacitors are usually specified in microfarads rather than farads, this calculator takes capacitance in µF and converts it internally before applying the formula.
| Term | Meaning |
|---|---|
| Xc | Capacitive reactance, in ohms: the opposition a capacitor presents to alternating current at a given frequency. |
| f | Frequency of the AC signal, in hertz (cycles per second). |
| C | Capacitance, in farads (entered here in microfarads, µF, and converted automatically). |
The inputs explained
| Field | What to enter |
|---|---|
| Frequency (Hz) | The frequency of the AC signal or supply, in hertz. Mains power is typically 50 Hz or 60 Hz; audio and signal circuits often run much higher. |
| Capacitance (µF) | The capacitor's rated capacitance, in microfarads (µF), as printed on the component or datasheet. |
When to use it
Designing a filter circuit
Choosing a capacitor value for a high-pass or low-pass filter starts with working out the reactance at the target cutoff frequency, since that reactance needs to be comparable to the circuit's resistance for the filter to behave as intended.
Checking a capacitor's effect at mains frequency
A capacitor used across mains voltage (50 Hz or 60 Hz) will have a very different reactance to the same component used in a kilohertz-range signal circuit, worth confirming before assuming a component behaves the same way in both settings.
Comparing capacitive and inductive reactance in an AC circuit
At the resonant frequency of an LC circuit, capacitive and inductive reactance are equal; calculating Xc at a candidate frequency is the first step toward finding or checking that resonance point.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How capacitive reactance changes with frequency
A fixed 1 µF capacitor, across a range of frequencies.
| Frequency | Capacitive reactance (Xc) |
|---|---|
| 50 Hz | 3,183.10 Ω |
| 60 Hz | 2,652.58 Ω |
| 100 Hz | 1,591.55 Ω |
| 1,000 Hz | 159.155 Ω |
| 10,000 Hz | 15.915 Ω |
| 100,000 Hz | 1.592 Ω |
How capacitive reactance changes with capacitance
A fixed 1,000 Hz signal, across a range of capacitor values.
| Capacitance | Capacitive reactance (Xc) |
|---|---|
| 0.01 µF | 15,915.49 Ω |
| 0.1 µF | 1,591.55 Ω |
| 1 µF | 159.155 Ω |
| 10 µF | 15.915 Ω |
| 100 µF | 1.592 Ω |
| 1000 µF | 0.1592 Ω |
Questions
What is the capacitive reactance formula?
Xc = 1 ÷ (2πfC), where f is frequency in hertz and C is capacitance in farads. The result, Xc, is in ohms, the same unit used for resistance.
Why does capacitive reactance fall as frequency rises?
A capacitor stores charge by having current flow onto and off its plates. At higher frequencies, the voltage across it reverses more often, so the capacitor spends less time fully charged and more time actively passing current, which shows up as lower opposition to the AC signal.
Is capacitive reactance the same as resistance?
They share the same unit (ohms) and both oppose current, but reactance is frequency-dependent and does not dissipate energy as heat the way resistance does; a capacitor stores and releases energy each cycle rather than consuming it.
How does capacitive reactance relate to an RC circuit's time constant?
Both come from the same resistance and capacitance values, but describe different things: the RC time constant (τ = RC) describes how quickly a capacitor charges or discharges through a resistor, while capacitive reactance describes its ongoing opposition to a continuous AC signal at a chosen frequency. See the RC time constant calculator for the charging and discharging behaviour.
For how a capacitor charges and discharges through a resistor over time, see the RC circuit charge and discharge calculator. For capacitors wired together rather than a single component, see the capacitors in series and parallel calculator.