Capacitive Reactance Calculator
Enter a frequency and a capacitance to get the capacitive reactance in ohms — and see why a capacitor blocks low frequencies but passes high ones.
Reactance in one step
Enter the signal frequency and the capacitance and the calculator returns the capacitive reactance (Xc = 1 / 2πfC) in ohms.
Use SI units
Frequency in hertz and capacitance in farads give reactance in ohms — a 10 µF capacitor is 0.00001 F, so convert microfarads before you start.
What is capacitive reactance?
A capacitor's opposition to AC
The capacitive reactance calculator finds how strongly a capacitor opposes an alternating current, measured in ohms. Reactance is the AC equivalent of resistance, but unlike a resistor it depends on frequency: the faster the signal swings, the more easily a capacitor passes it, so its reactance falls. This is why capacitors block steady (low-frequency) signals while letting high-frequency ones through — the principle behind coupling capacitors, filters, and crossover networks. Enter the signal frequency in hertz and the capacitance in farads to get the reactance in ohms.
Enter a frequency in hertz and a capacitance in farads to get the capacitive reactance in ohms instantly.
Capacitive reactance is one divided by the product of 2π, the frequency, and the capacitance.
Xc = 1 / (2π × f × C)Take a 10 µF capacitor (0.00001 F) on a 60 Hz mains supply. The denominator is 2π × 60 × 0.00001 = 0.0037699, and one divided by that gives 265.26 Ω. Because frequency sits in the denominator, doubling it to 120 Hz halves the reactance to about 133 Ω: the same capacitor opposes a fast signal far less than a slow one. Larger capacitors also have lower reactance at any given frequency, since capacitance is in the denominator too.
The formula is exact for an ideal capacitor, but a couple of practical points are worth keeping in mind.
Ideal capacitor and consistent units
This calculator gives the reactance of an ideal capacitor — it ignores equivalent series resistance (ESR), leakage, and lead inductance, which matter at very high frequencies. Reactance is also a magnitude only; in a full AC analysis it carries a −90° phase. Keep your units consistent — hertz for frequency and farads for capacitance — so convert microfarads (µF) and nanofarads (nF) to farads before you enter them, or the ohms will be wrong.