Capacitive Reactance Calculator — X = 1/(2πfC) | Metric & American Units

What is Capacitive Reactance?

Reactance is a property of an electric circuit element to oppose the flow of current. Using this definition, we can say that the capacitive reactance is like capacitor resistance. Even the reactance unit is the same as the resistance — the Ohm (Ω). Typically, we denote a reactance as X.

Although both the reactance (X) and the resistance (R) tend to be the same thing in a circuit, there is a particular distinction between them. The reactance influences the alternating current (AC), while the resistance affects the direct current (DC). In general, they are the components of an impedance Z, a complex quantity that determines the total opposition of a circuit to the current flow:

Z = R ± j × X
where j = √−1 is an imaginary unit

The capacitive reactance is a property of a capacitor. Similarly, inductive reactance is a property of an inductor. An ideal resistor has zero reactance, while it is a purely resistive element. On the contrary, perfect capacitors and inductors have zero resistance. So, strictly speaking, there is no such thing as capacitor resistance — we usually treat this phrase as a mental shortcut for capacitive reactance.

How to Calculate Capacitive Reactance

To calculate the capacitive reactance, follow these steps:

1. Write down the capacitance of the capacitor C (in Farads) and the AC frequency f (in Hz).
2. Replace in the capacitive reactance equation:

   X_C = 1 / (2π × f × C)

3. To use angular frequency instead (ω = 2π × f), use the following formula:

   X_C = 1 / (ω × C)

As mentioned above, capacitive reactance is a capacitor's property that opposes alternating current. The same is true for any set of capacitors that we can arrange in series or parallel.

How to Use the Capacitive Reactance Calculator

1. Choose the input mode:
   • Standard — enter frequency (f) and capacitance (C).
   • Angular Frequency (ω) — enter angular frequency (ω = 2πf) and capacitance.
2. Enter the capacitance and select the unit: F, mF, μF, nF, or pF. Most common capacitors are measured in microfarads (μF), nanofarads (nF), or picofarads (pF).
3. Enter the frequency and choose the unit: Hz, kHz, MHz, or GHz. Common mains electricity runs at 50 Hz (Europe) or 60 Hz (USA). Radio-frequency circuits typically operate in the kHz–GHz range.
4. Click Calculate — the result shows the capacitive reactance in Ohms (Ω), with automatic conversion to kΩ, MΩ, or other convenient units.

Unit Systems

This calculator supports both the metric (SI) and American engineering unit systems. In electrical engineering, both systems use the same SI units for capacitance (Farads), frequency (Hertz), and reactance (Ohms) — so there is no conversion gap between the two. The multiple unit options (pF, nF, μF, mF, F for capacitance; Hz, kHz, MHz, GHz for frequency) are provided to cover the full range of practical circuit scenarios.

FAQs

What happens to capacitive reactance as frequency increases?

Capacitive reactance is inversely proportional to frequency. As frequency increases, X_C decreases — a capacitor passes high-frequency AC more easily and blocks DC completely (f = 0 → X_C = ∞).

What happens to capacitive reactance as capacitance increases?

Similarly, capacitive reactance is inversely proportional to capacitance. A larger capacitor offers less opposition to alternating current at the same frequency.

Is capacitive reactance the same as resistance?

No. Resistance dissipates energy as heat. Reactance stores and returns energy — there is no energy loss in an ideal capacitor. Both are measured in Ohms, but they behave differently in a circuit. Together, R and X form the impedance Z of the circuit.

What is the difference between capacitive and inductive reactance?

Capacitive reactance (X_C) decreases with frequency, while inductive reactance (X_L = 2πfL) increases with frequency. At the resonant frequency, X_C = X_L and they cancel each other out, leaving only the resistance in the circuit.

Can I calculate reactance for capacitors in series or parallel?

Yes. For capacitors in series, the total capacitance decreases (1/C_total = 1/C₁ + 1/C₂ + …), increasing total reactance. For capacitors in parallel, total capacitance increases (C_total = C₁ + C₂ + …), decreasing total reactance.