Capacitive Reactance Calculator

What the capacitive reactance calculator does

A capacitor opposes alternating current by an amount that depends on the frequency, called its capacitive reactance. This calculator finds that reactance from the capacitance and the frequency, and can also show the angular frequency.

Below is what capacitive reactance is, the equation behind it, why it falls as frequency rises, and a worked example.

How to use it

1. Enter the capacitance, choosing its unit.
2. Enter the frequency of the alternating signal.
3. Press Calculate for the capacitive reactance, or Reset to clear it.

What capacitive reactance is

Capacitive reactance is the opposition a capacitor presents to alternating current. Like resistance, it is measured in ohms and limits the flow of current, but unlike resistance it depends on the frequency of the signal. A capacitor lets alternating current pass more easily at high frequencies and resists it strongly at low frequencies, blocking direct current entirely once it is fully charged. The reactance is the number that captures this frequency-dependent opposition at any given frequency.

The behaviour comes from how a capacitor works. It stores charge, and an alternating voltage constantly charges and discharges it, which keeps current flowing in the circuit. The faster the voltage alternates, the more readily this back-and-forth charging carries current, so the opposition is lower. At low frequencies the capacitor has time to charge up and choke off the current, so the opposition is high. This frequency dependence makes capacitive reactance central to how capacitors behave in alternating-current circuits and filters, and this calculator computes it from the capacitance and frequency.

The equation it uses

Capacitive reactance is one divided by the product of the angular frequency and the capacitance:

X_C = 1 ÷ (2πfC)

Here X_C is the capacitive reactance, f is the frequency, and C is the capacitance, with the factor of two pi converting the frequency into an angular frequency. Both the frequency and the capacitance sit in the denominator, so increasing either one lowers the reactance. The calculator evaluates this directly, and can also report the angular frequency, two pi times the frequency, which is the form often used in the underlying alternating-current mathematics.

Why it falls as frequency rises

The defining feature of capacitive reactance is that it decreases as frequency increases, the opposite of what many people first expect. At very low frequencies, approaching direct current, the reactance becomes very large, and for true direct current it is effectively infinite, which is why a capacitor blocks direct current once charged. As the frequency climbs, the reactance steadily falls, letting more current through.

This is why capacitors are described as passing high frequencies and blocking low ones. The effect is widely used: a capacitor can let a high-frequency signal through while stopping a steady direct-current voltage, which is the basis of coupling between circuit stages and of filters that separate signals by frequency. The capacitance matters too, with larger capacitors having lower reactance at any given frequency. Understanding that reactance drops with frequency is the key to understanding how capacitors shape signals, and the calculator makes the relationship concrete by computing the reactance at whatever frequency you choose.

Reactance is not resistance

Although capacitive reactance is measured in ohms and opposes current like resistance, it is fundamentally different in one important way: it does not dissipate energy. A resistor turns electrical energy into heat as current flows through it, but an ideal capacitor stores energy in its electric field and gives it back, rather than burning it off. So while reactance limits current, it does not waste power the way resistance does.

This distinction matters in alternating-current circuits, where the total opposition combines resistance and reactance into a quantity called impedance. The reactive part shifts the timing between voltage and current rather than consuming energy, which is connected to the idea of power factor. For the purpose of this calculator, the key point is that capacitive reactance is the energy-storing kind of opposition, frequency-dependent and lossless, distinct from the energy-dissipating resistance of a resistor. Keeping the two ideas separate helps make sense of how capacitors behave in real circuits.

Units and precision

The calculator takes the capacitance in farads or its smaller multiples, from millifarads down to picofarads, and the frequency in units from hertz up to terahertz, returning the capacitive reactance in ohms and its multiples. It applies the reactance relationship exactly, and an advanced mode also reports the angular frequency. The result is the opposition the capacitor presents at the frequency you enter, which will be lower at higher frequencies.

A worked example

Suppose a 1-microfarad capacitor carries a signal at 1 kilohertz.

The capacitive reactance is X_C = 1 ÷ (2πfC) = 1 ÷ (2π × 1000 × 0.000001) ≈ 159 ohms. Raise the frequency to 10 kilohertz and the reactance drops to about 16 ohms, ten times lower, since reactance is inversely proportional to frequency. At very low frequencies, the same capacitor would present a very high reactance, approaching a block for direct current.

Questions people ask

How do you calculate capacitive reactance?

Use X_C = 1 ÷ (2πfC), from the frequency and the capacitance. Both are in the denominator, so larger values give lower reactance.

Why does capacitive reactance fall with frequency?

Because a capacitor charges and discharges more readily at high frequency, carrying more current. At low frequency it chokes off current, and it blocks direct current entirely.

Is reactance the same as resistance?

Both are in ohms and oppose current, but reactance stores and returns energy rather than dissipating it as heat, and it depends on frequency, while resistance does not.

What is the reactance for direct current?

Effectively infinite. At zero frequency the formula gives an unbounded reactance, which is why a fully charged capacitor blocks direct current.

References

A quick note on where the physics comes from. Capacitive reactance and alternating-current behaviour are standard physics, set out in OpenStax's University Physics and in Georgia State University's HyperPhysics. The units follow NIST. The HyperPhysics link is worth a quick click to confirm it lands where you expect.

1. OpenStax, University Physics Volume 2, Section 15.3, RLC Series Circuits with AC. https://openstax.org/books/university-physics-volume-2/pages/15-3-rlc-series-circuits-with-ac
2. HyperPhysics, Capacitive Reactance. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/react.html
3. National Institute of Standards and Technology (NIST), SP 811, Guide for the Use of the International System of Units. https://www.nist.gov/pml/special-publication-811