Capacitive Reactance Calculator (2024)

This is the capacitive reactance calculator – a great tool that helps you estimate the so-called resistance of a capacitor in an electric circuit. You can find the capacitive reactance formula in the text below, and we explain why the reactance occurs for alternating current but not direct current.

If you're looking for a capacitance formula, feel free to try our capacitance calculator, but if you want to know how to calculate the capacitive reactance, you're at the right place – let's go!

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 number (square root of a negative number).

The capacitive reactance is a property of a capacitor. Similarly, inductive reactance is a property of an inductor – check the inductive reactance calculator for a more detailed explanation and formulas. An ideal resistor has zero reactance, while it's 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.

To calculate the capacitive reactance, follow these steps:

1. Write down the capacitance of the capacitor C and the AC frequency.

2. Replace in the capacitive reactance equation:

   X = 1 / (2 × π × f × C)

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

   X = 1 / (ω × C)

As you can see, the higher the frequency of the capacitance, the lower the reactance. Does it make sense?

Absolutely! Remember that a capacitor stores electric energy. While charging, it looks like the capacitor passes the current almost freely. The more it can absorb (the higher the capacity), the less it resists letting the current flow. Additionally, when the AC frequency gets higher, there is less time for the capacitor to charge fully. In the case of DC (f = 0), the capacitor initially charges, but then (in the equilibrium state), it acts as an open circuit.

There is nothing challenging about estimating the capacitive reactance of any capacitor. Let's practice the computations with an example.

Let's say we have a circuit with a spherical capacitor of capacitance C = 30 nF. We apply a voltage source, alternating with the frequency f = 60 Hz. What is the capacitive reactance in this circuit?

1. Convert the unit of the capacitance to Farads. We can use scientific notation to write the values compactly:

   C = 30 nF = 3×10⁻⁸ F

2. Work out the product of all the values in the denominator from the capacitive reactance formula:

   2 × π × f × C = 2 × π × 60 × 3×10⁻⁸ = 1.131×10⁻⁵

3. Find its multiplicative inverse, which is the ratio of 1 and our product: 1 / 1.131×10⁻⁵ = 88,419.41 Ω. Don't forget about the unit of reactance!

4. Write the result using an appropriate prefix:

   X = 88.41941 kΩ

5. Let's round the outcome to four significant figures:

   X = 88.42 kΩ

6. Check the result with our capacitive reactance calculator! Wow, relatively painless, wasn't it?

Ohms (Ω). XC refers to capacitive reactance and measures the opposition of current within an AC circuit. Its units are ohms, the same as resistance in DC circuits.