Understanding Capacitive Reactance
Capacitive reactance (Xc) is the opposition a capacitor presents to alternating current, measured in ohms just like resistance. Unlike a resistor, a capacitor's opposition to current changes with the frequency of the signal: it blocks low-frequency and DC current while letting high-frequency current pass with little opposition. This calculator applies the standard AC circuit formula to any frequency and capacitance you enter.
The formula
Capacitive reactance is calculated as:
Xc = 1 / (2πfC)
where f is the signal frequency in hertz (Hz) and C is the capacitance in farads (F). The term 2πf is the angular frequency ω, so the formula is often written Xc = 1/(ωC). Because f and C are both in the denominator, Xc shrinks as either frequency or capacitance increases, and grows without bound as either approaches zero.
Working with units
- Frequency is entered in hertz (Hz) — cycles per second.
- Capacitance is entered in your chosen unit and converted to farads internally: 1 µF = 10-6 F, 1 nF = 10-9 F, 1 pF = 10-12 F.
- The result is reported in ohms (Ω), automatically scaled to kilo-ohms (kΩ) or mega-ohms (MΩ) when the value is large, or milliohms (mΩ) when it is very small.
Knowing the limits
This formula assumes an ideal capacitor operating in sinusoidal steady-state AC — no equivalent series resistance (ESR), no lead inductance, and a single fixed frequency. Real capacitors deviate from ideal behavior at very high frequencies, where parasitic inductance can make them behave inductively instead of capacitively. For a full picture of an AC branch, combine Xc with any resistance R using the impedance formula Z = √(R² + Xc²).
Frequently Asked Questions
What is capacitive reactance?
How does frequency affect capacitive reactance?
What units does this calculator use?
How is capacitive reactance different from resistance?
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Common Measurement Conversions for Everyday Use
| Quantity | US / Imperial | Metric Equivalent |
|---|---|---|
| 1 cup | 8 fl oz | 237 mL |
| 1 pint | 16 fl oz | 473 mL |
| 1 quart | 32 fl oz | 946 mL |
| 1 gallon | 128 fl oz | 3.785 L |
| 1 lb | 16 oz | 453.6 g |
| 1 ft | 12 in | 30.48 cm |