Capacitive Reactance Calculator

Capacitive reactance calculator for frequency and capacitor sizing

Capacitive reactance (often written as Xc) is the opposition a capacitor presents to alternating current (AC). Unlike a resistor, a capacitor does not oppose current the same way at all frequencies. It changes with frequency and capacitance. At higher frequencies a capacitor allows more AC current to pass, so its reactance drops. At lower frequencies the reactance rises, which means the capacitor behaves more like an open circuit.

This calculator helps you convert a capacitor value and an AC frequency into capacitive reactance in ohms. That is the most common need when you are checking how a capacitor will behave in an AC circuit, estimating current draw in a simple series scenario, or sanity checking a capacitor choice for filtering or coupling. It is also useful for quick comparisons: changing the capacitance by 10 times or changing the frequency by 10 times changes Xc by the opposite factor.

If you also know the AC voltage (RMS), this calculator can estimate the AC current that would flow if the capacitor were the only significant impedance in series. That is a simplified case, but it is still useful when you are evaluating things like a capacitive dropper concept, the expected current through a capacitor at mains frequency, or whether a capacitor will significantly load a signal source. When voltage is provided, the calculator also reports reactive power (VAR), which is often relevant in AC analysis because capacitors exchange energy with the source each cycle.

Assumptions and how to use this calculator

• The reactance formula used is Xc = 1 / (2π f C) with f in hertz and C in farads.
• The optional current estimate assumes a simple series path where the capacitor dominates the impedance and other resistances or inductances are ignored.
• Voltage input is treated as RMS voltage. If you only have peak voltage, convert it to RMS before using this calculator.
• Real capacitors have tolerance, leakage, and equivalent series resistance (ESR). This calculator treats the capacitor as ideal for the reactance calculation.
• At very high frequencies, parasitics and layout effects can dominate. Treat results as a starting estimate, not a measurement substitute.

Common questions

What is capacitive reactance in simple terms?

Capacitive reactance is how much a capacitor resists changes in voltage in an AC circuit, expressed as an equivalent resistance in ohms. A high Xc means the capacitor allows little AC current to flow at that frequency. A low Xc means the capacitor allows more AC current to flow.

Why does Xc change with frequency?

A capacitor stores and releases energy as the AC waveform changes. When the frequency increases, the voltage changes faster, and the capacitor can pass more alternating current for the same capacitance. The formula reflects this: doubling frequency halves the reactance.

Does capacitive reactance equal impedance?

For an ideal capacitor by itself, the magnitude of its impedance equals Xc. In real circuits, total impedance can include resistance and inductance as well. If your circuit has multiple components, Xc is only one part of the overall impedance.

What if I do not know the exact capacitor value?

You can still get a useful estimate by using the nominal value printed on the capacitor. Many capacitors have wide tolerances, so the true value may differ. If accuracy matters, run the calculation again using a low and high estimate based on tolerance to see a practical range for Xc.

Why is the current estimate only optional?

Current depends on the full circuit, not only the capacitor. The optional current is most useful when the capacitor is effectively the main limiting element in series. If there are other significant resistances or inductances, the true current will be lower or behave differently across frequency.