Capacitive Reactance Calculator
Calculate capacitive reactance (Xc) for a capacitor in an AC circuit at a given frequency. Used by engineers and students analyzing filters, tuned circuits, and impedance matching.
About this calculator
Capacitive reactance (Xc) describes how strongly a capacitor opposes the flow of alternating current at a given frequency. The formula is Xc = 1 / (2π × f × C), where f is the frequency in hertz (Hz) and C is the capacitance in farads (F). Because capacitance is commonly given in microfarads (μF), the calculator converts it using C(F) = C(μF) / 1,000,000 before applying the formula: Xc = 1 / (2 × π × f × (C_μF / 1,000,000)). Reactance is measured in ohms (Ω), just like resistance, but unlike resistance it varies with frequency. As frequency increases, Xc decreases — a capacitor passes high-frequency signals more easily and blocks DC entirely. This principle is fundamental to designing high-pass filters, coupling networks, and resonant circuits.
How to use
Suppose you have a 100 μF capacitor operating in a 60 Hz AC circuit. First convert capacitance: 100 μF = 100 / 1,000,000 = 0.0001 F. Then apply the formula: Xc = 1 / (2 × π × 60 × 0.0001) = 1 / (0.037699) ≈ 26.53 Ω. So at 60 Hz, this capacitor presents about 26.53 Ω of reactance to the circuit. Enter your frequency and capacitance into the calculator to get the reactance instantly and use the result in impedance or filter design calculations.
Frequently asked questions
What is capacitive reactance and how does it affect an AC circuit?
Capacitive reactance (Xc) is the opposition a capacitor offers to alternating current, measured in ohms. Unlike resistance, it is not constant — it depends on both the capacitance value and the signal frequency. A high Xc means the capacitor blocks most of the AC signal, while a low Xc allows it to pass freely. This frequency-dependent behavior makes capacitors essential in filters, tone controls, and signal coupling circuits.
How does frequency affect capacitive reactance?
Capacitive reactance is inversely proportional to frequency: as frequency goes up, Xc goes down. At very low frequencies (approaching DC), Xc becomes extremely large, effectively blocking current. At high frequencies, Xc approaches zero, allowing current to flow almost unimpeded. This is why capacitors are used as high-pass filter elements — they pass high-frequency signals and attenuate low-frequency ones, including DC.
How do I use capacitive reactance in an impedance or filter design calculation?
In AC circuits, capacitive reactance is treated like resistance in Ohm's Law: V = Xc × I. To design a simple RC high-pass filter, you choose a capacitor and resistor such that the cutoff frequency fc = 1 / (2π × R × C) falls where you want the filter to transition. At that frequency, Xc equals R, meaning the signal is attenuated by 3 dB. Knowing Xc also lets you compute total impedance in series or parallel RLC circuits using Z = √(R² + Xc²).