Reactance calculator
Pick a capacitor or an inductor, enter its value and the frequency, and see its reactance, the opposition it offers to alternating current. Add the supply voltage to see the current it draws. Works from mains 50 or 60 Hz up to gigahertz.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose a capacitor or an inductor.
- Enter the frequency and its unit.
- Enter the capacitance (pF to mF) or the inductance (µH to H).
- Optionally enter the RMS voltage to see the current; leave 0 to skip it.
- Capacitive reactance: Xc = 1 ÷ (2π × f × C).
- Inductive reactance: XL = 2π × f × L.
- Current: I = V ÷ X. Susceptance: B = 1 ÷ X, in siemens.
Worked example
A 10 µF capacitor on 230 V, 50 Hz mains: Xc = 1 ÷ (2π × 50 × 0.00001) = 1 ÷ 0.0031416 = 318.3099 Ω, so it draws 230 ÷ 318.31 = 0.722566 A.
A 100 mH inductor at the same 50 Hz: XL = 2π × 50 × 0.1 = 31.4159 Ω, and the current at 230 V is 7.321127 A.
A 100 nF capacitor at 1 kHz: Xc = 1 ÷ (2π × 1000 × 0.0000001) = 1,591.5494 Ω.
Tips
- Units first. Most errors come from a µF typed as nF: a factor of 1000 in the answer.
- Reactance is not resistance. Voltage and current in an ideal capacitor or inductor are 90° out of phase, so the power it takes on average is zero.
- Filters. For a simple RC filter the cut-off is where Xc equals R; the RC time constant calculator gives it directly.
Frequently asked questions
What is reactance?
Reactance is the opposition of a capacitor or an inductor to alternating current, measured in ohms like resistance. Unlike a resistor, it does not turn energy into heat; it stores energy and gives it back every cycle, and it changes with frequency.
How does reactance change with frequency?
A capacitor's reactance Xc = 1 ÷ (2πfC) falls as the frequency rises, so capacitors pass high frequencies and block DC. An inductor's reactance XL = 2πfL rises with frequency, so inductors pass DC and choke high frequencies.
Where do Xc and XL become equal?
At the resonant frequency f = 1 ÷ (2π√(LC)). The LC resonance calculator finds it, together with the Q factor and bandwidth.
Is the current here the real current from the wall?
It is the current through an ideal capacitor or inductor connected alone across that voltage, I = V ÷ X. Real parts add some resistance, and in a circuit with resistors you need the total impedance Z = √(R² + X²).