Capacitive Reactance Calculator

Capacitive reactance (Xc) is how strongly a capacitor opposes alternating current, in ohms. It falls as the frequency or the capacitance rises: a capacitor lets high frequencies through and holds back low ones. To direct current, at 0 Hz, it is an open circuit.

Xc = 1 / (2 * π * f * C)

where f is the frequency in hertz and C the capacitance in farads. Enter any two of the three, and the calculator finds the third:

• C = 1 / (2 * π * f * Xc)
• f = 1 / (2 * π * C * Xc)

Examples

1. A 10 µF capacitor at 1 kHz: 10 µF is 0.00001 F.

   Xc = 1 / (2 * π * 1000 * 0.00001) = 15.92 Ω

2. The capacitor for 50 Ω at 500 Hz:

   C = 1 / (2 * π * 500 * 50) = 6.366 µF

3. The frequency where 100 nF has 1 kΩ:

   f = 1 / (2 * π * 0.0000001 * 1000) = 1,592 Hz

Each field has its own unit menu, so values can be entered in µF, nF, kHz or kΩ directly.

Frequently asked questions

What is the total reactance of capacitors in parallel or in series?

Work out the total capacitance first. In parallel, capacitances add: 20 µF and 30 µF make 50 µF, which at 400 Hz is 7.958 Ω. In series, 1 / C = 1 / C1 + 1 / C2, so the same two make 12 µF.

What is the difference between reactance and impedance?

Reactance is the opposition of a capacitor or inductor alone. Impedance (Z) combines it with resistance: for a resistor and capacitor in series, Z = √(R² + Xc²). In a capacitor, the current leads the voltage by 90°.

How is it related to inductive reactance?

An inductor does the opposite, with XL = 2 * π * f * L rising with frequency. Where the two are equal, a circuit resonates. See the Inductive Reactance Calculator and the Resonant Frequency Calculator.