Reactance Calculator
Find capacitive or inductive reactance at any frequency, with unit-aware inputs and the math shown.
At 1 kHz, this capacitor opposes AC with 159.15 Ω. Reactance falls as frequency rises.
Reactance is the ohms of opposition a capacitor or inductor gives to AC. Capacitive reactance is Xc = 1 ÷ (2πfC); inductive reactance is XL = 2πfL. A 1 µF cap at 1 kHz gives Xc = 1 ÷ (2π × 1000 × 0.000001) = 159.15 Ω. Xc falls as frequency rises, while XL rises with frequency.
What reactance is
Reactance is the frequency-dependent opposition that capacitors and inductors present to alternating current, measured in ohms (Ω) just like resistance. The difference is that reactance changes with frequency and stores energy rather than dissipating it as heat. A capacitor charges and discharges each cycle, so it passes fast signals easily but blocks slow ones — its reactance Xc drops as frequency climbs. An inductor resists changes in current, so it passes low frequencies and chokes high ones — its reactance XL grows with frequency. Together these behaviours let designers build filters, tuned circuits, and coupling networks.
f = frequency (Hz), C = capacitance (F), L = inductance (H); reactance is in ohms (Ω). 2πf is the angular frequency ω.
Worked example
Find the reactance of a 1 µF capacitor at 1 kHz, then a 10 mH inductor at the same frequency:
- 1 Convert to base units. Put frequency in hertz and the component in farads or henries: 1 kHz = 1000 Hz, 1 µF = 0.000001 F, 10 mH = 0.01 H.
- 2 Pick the right formula. Use Xc = 1 ÷ (2πfC) for a capacitor, or XL = 2πfL for an inductor.
- 3 Compute capacitive reactance. Xc = 1 ÷ (2π × 1000 × 0.000001) = 1 ÷ 0.0062832 = 159.15 Ω.
- 4 Compute inductive reactance. XL = 2π × 1000 × 0.01 = 62.83 Ω.
- 5 Combine with resistance if needed. Impedance magnitude is Z = √(R² + X²); reactance alone assumes an ideal, lossless component.
How reactance changes with frequency
Capacitive reactance is inversely proportional to frequency; inductive reactance is directly proportional.
| Change | Capacitive Xc = 1 ÷ (2πfC) | Inductive XL = 2πfL |
|---|---|---|
| Frequency × 10 | Xc ÷ 10 (falls) | XL × 10 (rises) |
| Frequency × 2 | Xc ÷ 2 (falls) | XL × 2 (rises) |
| At DC (f → 0) | Xc → ∞ (blocks) | XL → 0 (passes) |
| At very high f | Xc → 0 (passes) | XL → ∞ (blocks) |
| Component × 2 | Xc ÷ 2 | XL × 2 |
Reactance, resistance, and impedance
Reactance is frequency-dependent opposition; resistance is not. A resistor drops the same ohms at every frequency and turns electrical energy into heat. A capacitor or inductor instead stores and returns energy each cycle, and the ohms it presents depend on frequency — which is exactly why reactance appears in filters, crossovers, and tuned radio circuits.
Impedance combines the two. In a real circuit with both resistance and reactance, the total opposition is the impedance Z, whose magnitude is √(R² + X²). Reactance also shifts the phase between voltage and current by ±90°, whereas resistance keeps them in step. Use reactance on its own for an ideal component, and impedance when resistance is in the picture.