Capacitive Reactance Calculator
Find capacitive reactance from frequency and capacitance.
Results update as you type.
How it works
A capacitor opposes alternating current with a frequency-dependent reactance, and this calculator computes it. You enter the frequency and the capacitance, and it returns the capacitive reactance in ohms. The reactance equals one divided by two pi times the frequency times the capacitance. Unlike a resistor, whose opposition is constant, a capacitor's reactance falls as frequency rises: it strongly opposes low-frequency and direct current (infinite reactance at zero frequency, blocking DC) but passes high-frequency signals with little opposition. Larger capacitance also lowers reactance at a given frequency. This frequency-dependent behavior is what makes capacitors so useful in electronics: they block DC while passing AC (coupling), filter out unwanted frequencies (in high-pass and low-pass filters), smooth power supply ripple, and set frequencies in tuned and timing circuits. Reactance combines with resistance to form impedance, the total opposition in an AC circuit, and it causes the current through a capacitor to lead the voltage by 90 degrees. Understanding capacitive reactance is fundamental for designing filters, coupling and decoupling networks, and AC circuits generally. The calculator uses capacitance in microfarads, a common unit, and frequency in hertz. Both must be positive. Enter the frequency and capacitance to find the capacitive reactance.
Xc = 1 / (2πfC)
Capacitive reactance is the opposition a capacitor presents to alternating current. It equals one divided by the product of two pi, the frequency, and the capacitance. Higher frequency or larger capacitance means lower reactance, so a capacitor passes high frequencies more easily than low ones.
Source:Electrical engineering
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter frequency and capacitance.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Capacitive reactance updates instantly as you type — no button to press.
Frequently asked questions
What is capacitive reactance?
Capacitive reactance is the opposition a capacitor offers to alternating current, measured in ohms. It equals one divided by two pi times the frequency times the capacitance. Unlike resistance, it depends on frequency: it is high at low frequencies, blocking direct current entirely, and low at high frequencies, letting them pass easily.
Why does reactance decrease with frequency?
A capacitor charges and discharges as AC voltage alternates. At higher frequencies it charges and discharges faster, allowing more current to flow for the same voltage, which means less opposition, or lower reactance. At very low frequencies or DC, it fully charges and blocks further current, giving very high or infinite reactance.
How is capacitive reactance used in filters?
Because reactance changes with frequency, capacitors are key filter components. In a high-pass filter, a capacitor passes high frequencies and blocks low ones; in a low-pass filter, it does the reverse when arranged differently. This frequency selectivity lets designers separate or combine signals, remove noise, and shape frequency responses.
What is the difference between reactance and impedance?
Reactance is the opposition from a capacitor or inductor alone, which depends on frequency and stores energy rather than dissipating it. Impedance is the total opposition in an AC circuit, combining resistance (which dissipates energy) and reactance as a complex quantity. In a circuit with both, impedance is found by combining resistance and reactance.
Why does current lead voltage in a capacitor?
In a capacitor, the current is greatest when the voltage is changing fastest, which occurs a quarter cycle before the voltage peaks. As a result, the current waveform leads the voltage waveform by 90 degrees. This phase relationship, combined with the frequency-dependent reactance, defines how capacitors behave in AC circuits.