When frequency decreases, capacitive reactance increases. This is an inverse relationship: halve the frequency and the capacitive reactance doubles. At the extreme, when frequency drops to zero (pure direct current), capacitive reactance becomes infinite, meaning the capacitor blocks current flow entirely.
The formula that governs this is Xc = 1 / (2πfC), where Xc is capacitive reactance in ohms, f is frequency in hertz, and C is capacitance in farads. Because frequency sits in the denominator, any decrease in f makes the overall value of Xc larger.
Why Lower Frequencies Face More Opposition
A capacitor stores energy by accumulating charge on two plates separated by an insulating material. In an AC circuit, the voltage constantly reverses direction, so the capacitor is always charging and discharging. At high frequencies, this reversal happens so quickly that current flows almost continuously through the circuit as the capacitor never fully charges before the voltage flips again. The capacitor barely resists the current because it never “fills up.”
At lower frequencies, each half of the AC cycle lasts longer. The capacitor has more time to charge toward the supply voltage, and as it does, it opposes more and more of the incoming current. By the time the voltage reverses, the capacitor has built up significant charge and has been pushing back against the source for a larger portion of the cycle. The result is less overall current flow, which is exactly what a higher reactance value describes.
At zero frequency, meaning steady DC, the capacitor charges fully to the supply voltage and then stops accepting current altogether. Its reactance is effectively infinite, making it an open circuit. This is why capacitors are often described as “blocking DC.”
A Quick Calculation
Suppose you have a 10 µF capacitor. At 1,000 Hz, its reactance is:
Xc = 1 / (2 × 3.14159 × 1000 × 0.00001) = about 15.9 ohms
Drop the frequency to 100 Hz and the same capacitor now has:
Xc = 1 / (2 × 3.14159 × 100 × 0.00001) = about 159 ohms
A tenfold decrease in frequency produced a tenfold increase in reactance. The relationship scales perfectly because it’s a straight inverse proportion.
How This Differs From Resistance
Ordinary resistance stays the same regardless of frequency. A 100-ohm resistor is 100 ohms whether you feed it 60 Hz power or a 10 kHz signal. Capacitive reactance changes with every shift in frequency, which makes it a fundamentally different kind of opposition to current.
There’s also a phase difference. In a purely capacitive circuit, current leads voltage by 90 degrees. This phase shift stays constant at any frequency. What changes is only the magnitude of the opposition, not the timing relationship between current and voltage.
When a circuit contains both resistance and capacitive reactance, the total opposition to current is called impedance. You can’t simply add resistance and reactance together because of that 90-degree phase difference. Instead, impedance is calculated using the Pythagorean relationship: Z equals the square root of (R² + Xc²). On an impedance diagram, resistance is plotted along the horizontal axis and capacitive reactance is plotted downward along the vertical axis. As frequency drops and Xc grows, the impedance vector stretches further in the vertical direction, increasing total impedance.
Real-World Limits at Low Frequencies
The formula Xc = 1/(2πfC) assumes a perfect capacitor, but real capacitors have a small amount of internal resistance called equivalent series resistance (ESR). For aluminum electrolytic capacitors, typical ESR values range from around 100 to 500 milliohms. At moderate and high frequencies, this internal resistance is negligible compared to the reactance. But as frequency drops very low and reactance climbs into the thousands or tens of thousands of ohms, the ESR barely matters because the capacitance itself dominates the impedance. ESR becomes most significant near a capacitor’s resonant frequency, not at the low-frequency end.
Where This Matters in Practice
The frequency-reactance relationship is the foundation of how audio crossovers and electronic filters work. In a high-pass filter, a capacitor is placed in series with the signal path. High-frequency signals pass through easily because the capacitor’s reactance is low. Low-frequency signals face high reactance and get attenuated. At the cutoff frequency, the signal drops to about 70.7% of its full amplitude (a gain of 1/√2). Below that frequency, the output falls off progressively.
Speaker crossover networks rely on this directly. A capacitor in series with a tweeter blocks low bass frequencies (where reactance is high) while allowing higher frequencies through. A typical two-way crossover at 2,000 Hz might use a 19.4 µF capacitor paired with a 0.33 mH inductor. The capacitor protects the tweeter by presenting enormous reactance to frequencies well below 2,000 Hz, preventing that energy from reaching a driver that can’t handle it.
Low-pass filters work the opposite way. A capacitor placed across the signal path to ground shorts high frequencies (low reactance) while leaving low frequencies intact (high reactance). At frequencies well below the cutoff, the capacitor’s reactance is so large it essentially disappears from the circuit, and the signal passes through at full strength.
Power supply filtering uses the same principle. Smoothing capacitors in a power supply are chosen to have low reactance at the ripple frequency (often 120 Hz in rectified AC). If the ripple frequency were lower, the same capacitor would have higher reactance and would do a poorer job of smoothing the output. Designers compensate by using larger capacitance values when filtering lower-frequency ripple.
The Key Takeaway in One Line
Capacitive reactance and frequency move in opposite directions. Lower frequency always means higher reactance, more opposition to current, and less signal getting through. The relationship is perfectly inverse and predictable from Xc = 1/(2πfC).