Capacitive Reactance Calculator
Calculate capacitor reactance using the formula XC = 1 / (2πfC). Enter frequency and capacitance values, select units, then click calculate.
What is the reactance of a capacitor?
Capacitive reactance is the opposition a capacitor gives to alternating current (AC). Unlike resistance, which dissipates energy as heat, reactance stores and releases energy in an electric field. In AC circuits, this behavior is frequency-dependent, which means the same capacitor can block low-frequency signals while allowing high-frequency signals to pass more easily.
The unit of capacitive reactance is ohms (Ω), and it is represented as XC. A larger XC means more opposition to AC current.
Formula used by this calculator
The calculator uses the standard equation:
XC = 1 / (2πfC)
- XC = capacitive reactance in ohms (Ω)
- f = frequency in hertz (Hz)
- C = capacitance in farads (F)
- π = 3.14159...
If frequency increases, reactance decreases. If capacitance increases, reactance also decreases. This inverse relationship is why capacitors are widely used in filtering and coupling applications.
How to use the calculator
Step-by-step
- Enter frequency value and select the correct frequency unit (Hz, kHz, or MHz).
- Enter capacitance value and select the correct capacitance unit (F, mF, µF, nF, or pF).
- Click Calculate Reactance to get XC.
- Use Reset to clear values and start over.
Tip: Most practical capacitor values in electronics are in microfarads (µF), nanofarads (nF), or picofarads (pF). Always double-check units to avoid errors by factors of 1,000 or 1,000,000.
Example calculations
Example 1: 100 µF capacitor at 60 Hz
XC = 1 / (2π × 60 × 100×10-6) ≈ 26.5 Ω
This relatively low reactance is why larger capacitors can pass low-frequency AC more easily.
Example 2: 10 nF capacitor at 1 kHz
XC = 1 / (2π × 1000 × 10×10-9) ≈ 15.9 kΩ
At the same frequency, a smaller capacitor produces much higher reactance.
Where capacitive reactance matters
- RC filters: Set cutoff behavior in low-pass and high-pass networks.
- Power supplies: Smooth ripple and reduce noise.
- Signal coupling: Block DC while passing AC content.
- Timing circuits: Work with resistors to define time constants.
- Audio electronics: Shape tone and frequency response.
Practical engineering notes
Real capacitors are non-ideal
Real capacitors include equivalent series resistance (ESR), leakage current, and parasitic inductance. At high frequencies, these effects can significantly alter expected reactance.
Tolerance and voltage rating
Capacitors come with tolerance ranges (e.g., ±5%, ±10%, ±20%) and maximum voltage ratings. For robust designs, account for worst-case capacitance and voltage stress.
Temperature dependence
Some dielectric materials shift capacitance with temperature. In precision analog or timing circuits, this can impact performance over operating range.
Quick FAQ
Does capacitive reactance apply to DC?
In steady-state DC (f = 0), XC tends to infinity, meaning an ideal capacitor behaves like an open circuit after charging.
Why does reactance drop at higher frequency?
Faster voltage changes cause more charge/discharge cycles per second, allowing more AC current through the capacitor.
Can I use this for design work?
Yes for first-pass calculations. For final design, include ESR, tolerance, dielectric behavior, and frequency response from component datasheets.
Conclusion
A capacitor’s reactance is one of the most important values in AC circuit analysis. With a quick calculation of XC, you can estimate current flow, filter behavior, and coupling performance. Use the calculator above to speed up design checks and validate hand calculations instantly.