Capacitive Reactance Calculator – Xc Formula, Free Online

How to use the capacitive reactance calculator

1. Enter the frequency (f) in hertz — for example, 50 for mains frequency or 1000 for a 1 kHz audio signal.
2. Enter the capacitance (C) in farads — note that most real capacitors are labelled in microfarads (µF) or nanofarads (nF), so convert first: 10 µF is 0.00001 farads.
3. Click “Calculate Reactance”.
4. Read the result in ohms, shown on the page.

A worked example

Suppose you have a 10 µF capacitor in a circuit running at 1,000 Hz. The formula is Xc = 1 ÷ (2 × π × f × C). Put the numbers in: 2 × π × 1,000 × 0.00001 = 0.0628, and 1 ÷ 0.0628 = 15.92. So the capacitive reactance is about 15.9 ohms. Try the same capacitor at 50 Hz and the answer jumps to about 318 ohms — the same part, nearly twenty times the opposition, simply because the frequency is lower. That is the key behaviour of capacitors: higher frequency means lower reactance, and lower frequency means higher reactance.

When capacitive reactance matters

• Audio and filter circuits: capacitors set the cutoff frequency of high-pass and low-pass filters, so Xc decides which frequencies pass and which are blocked.
• Power supplies: smoothing capacitors and coupling capacitors are chosen by working out their reactance at the operating frequency.
• Electronics study: students meet Xc in almost every AC circuits course, and homework problems usually give frequency and capacitance and ask for Xc in ohms.
• Troubleshooting: if a signal is weaker than expected, checking the reactance of the coupling capacitor at the signal frequency often explains why.

What the result means

Reactance is measured in ohms like resistance, but it is not the same thing as resistance. A resistor turns energy into heat; a capacitor stores energy and returns it to the circuit, shifting the timing of the current relative to the voltage by a quarter of a cycle. That is why Xc is sometimes called an opposition rather than a resistance. In a real circuit the total opposition, called impedance, combines resistance and reactance, and you cannot get it by simply adding the two numbers together. This calculator gives you the capacitive part only, which is exactly what most textbook and design questions ask for.

Honest limitations

• The calculator assumes an ideal capacitor. Real capacitors have small series resistance and inductance, which matter at very high frequencies.
• Watch your units: entering 10 instead of 0.00001 for a 10 µF capacitor gives a wildly wrong answer, so convert microfarads and nanofarads to farads first.
• The result is for one steady frequency. Music and other complex signals contain many frequencies at once, each with its own reactance.
• This tool finds Xc only. For the total impedance of a resistor-capacitor circuit, you also need the resistance, combined with the formula Z = √(R² + Xc²).

Capacitive reactance calculator – frequently asked questions

What is the formula for capacitive reactance?

Xc = 1 ÷ (2πfC), where f is the frequency in hertz and C is the capacitance in farads. The answer comes out in ohms.

Does reactance go up or down with frequency?

Down. Doubling the frequency halves the reactance. At zero frequency — direct current — a capacitor blocks the current completely, which is the same as infinite reactance.

What units should I enter?

Frequency in hertz and capacitance in farads. Convert microfarads (µF) by dividing by 1,000,000 and nanofarads (nF) by dividing by 1,000,000,000.

How is capacitive reactance different from inductive reactance?

They move in opposite directions: capacitive reactance falls as frequency rises, while inductive reactance rises. Try the inductive reactance calculator to compare the two, and the LC resonance calculator to see where they cancel each other out.