Capacitive Reactance Calculator

Created by Luciano Mino
Last updated: Aug 24, 2022

Our capacitive reactance calculator allows you to obtain the opposition to current flow introduced by a capacitor in an AC circuit.

If you don't know what capacitive reactance and impedance are, you've come to the right place. In this short text, we will cover:

  • Capacitive reactance definition (sometimes called capacitor resistance);
  • Capacitive reactance formula; and
  • How to easily calculate capacitive reactance.

What is capacitive reactance?

As a capacitor charges up in a DC circuit, the charges accumulating on the capacitor plates will begin to oppose the current flow until it reaches zero (see force between two charges).

In AC circuits, however, capacitors are constantly being charged and discharged, so this opposition to current is present at all times. We call this resistance to current flow the capacitive reactance, and we measure it in ohms Ω.

🙋 Keep in mind that, while they are similar, resistance only applies to resistors, while capacitive reactance is a property of capacitors. That's why "capacitor resistance" does not accurately describe this phenomenon.

Let's now see how to calculate the capacitive reactance.

How to calculate capacitive reactance

We can quickly obtain the capacitive reactance of a capacitor with the capacitive reactance formula:

X=12πfCX = \frac{1}{2 \pi f C}

where:

  • ff is the frequency of the power supply;
  • CC is the capacitor's capacitance; and
  • XX is the capacitive reactance, in ohms.

Alternatively, we can use the angular frequency ω\omega:

X=1wCX = \frac{1}{w C}

As you can see, increasing the frequency will decrease the capacitive reactance.

At the same time, increasing the capacitance of the capacitor will also lower its capacitive reactance.

Why? Remember what we discussed at the beginning: as a capacitor is being charged, it allows current to flow freely through it and gradually slows down when near fully charged.

If the capacitor is large enough or if we don't give it enough time to charge, the charges won't be able to pile up on the capacitor plates, and stop the current flow, thus increasing the current in the circuit!

Luciano Mino
Capacitance (C)
pF
Frequency (f)
Hz
Reactance (X)
Ω
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