Reactance, Filter & Resonance Calculator
Calculate capacitive and inductive reactance, RC and RL filter cutoff frequencies, and the resonant frequency of an LC circuit — with a log–log plot where the XC and XL curves cross at resonance.
Enter a frequency, then a capacitance and/or inductance to get the capacitive reactance XC and inductive reactance XL at that frequency.
About the Reactance, Filter & Resonance Calculator
In a DC circuit only resistance limits current, but the moment a signal changes with time — an AC waveform, an audio tone, an RF carrier — capacitors and inductors start to matter. Their opposition to alternating current is called reactance, and unlike resistance it depends on frequency. This calculator covers the three AC fundamentals the rest of the electrical cluster leaves out. First, reactance itself: the capacitive reactance XC = 1/(2πfC), which falls as frequency rises, and the inductive reactance XL = 2πfL, which climbs with frequency. Second, the passive filter cutoff: the −3 dB corner frequency fc = 1/(2πRC) of an RC filter or fc = R/(2πL) of an RL filter, the point where a low-pass or high-pass filter starts to roll off. Third, LC resonance: the frequency f0 = 1/(2π√(LC)) at which the inductive and capacitive reactances are equal and cancel, which is how radios tune, oscillators run and tank circuits ring. A single log–log plot ties it all together — the falling XC curve and the rising XL curve cross exactly at the resonant frequency, and the filter cutoff is simply where the reactance equals R. It is built for electronics students, ham-radio operators, audio and RF hobbyists and anyone designing crossovers, tuned circuits or signal filters.
How to use this calculator
- 1 Choose Reactance to find XC and XL at a frequency, Filter cutoff for an RC/RL corner frequency, or LC resonance for a tuned circuit.
- 2 In Reactance mode enter the frequency and a capacitance and/or inductance; in Filter mode pick RC or RL and enter R with C or L; in Resonance mode enter L and C.
- 3 Each field has its own SI-prefix unit (Hz/kHz/MHz, pF/nF/µF, µH/mH/H, Ω/kΩ/MΩ), so you can type real component values directly.
- 4 Read the result and study the log–log plot, where XC and XL cross at the resonant frequency and the filter cutoff sits where the reactance equals R.
How reactance, cutoff and resonance are calculated
Reactance is the frequency-dependent opposition of a capacitor or inductor to alternating current, measured in ohms just like resistance but shifting the current 90° out of phase with the voltage. Capacitive reactance: XC = 1 / (2πfC) A capacitor blocks DC (infinite reactance at 0 Hz) and passes high frequencies more and more easily, so XC falls as f rises. Inductive reactance: XL = 2πfL An inductor passes DC freely and opposes rapid change, so XL grows in direct proportion to frequency. Filter cutoff (−3 dB corner): RC filter: fc = 1 / (2πRC) RL filter: fc = R / (2πL) At the cutoff the reactance of the reactive element equals R, the output has dropped to about 70.7% (−3 dB) of the input, and the phase shift is 45°. Swapping which element is the output turns a low-pass filter into a high-pass one with the same corner frequency. LC resonance: f0 = 1 / (2π√(LC)) Resonance is the frequency where XL = XC. Setting 2πfL = 1/(2πfC) and solving for f gives f0. At that point the two reactances cancel: a series LC looks like a near short circuit and a parallel LC looks like a near open circuit, and both reactances equal the characteristic impedance √(L/C).
Frequently asked questions
What is capacitive reactance?
Capacitive reactance is the opposition a capacitor presents to alternating current, given by XC = 1/(2πfC) and measured in ohms. Unlike a resistor, that opposition depends on frequency: at DC (0 Hz) a capacitor is an open circuit with infinite reactance, and as the frequency rises XC falls, so the capacitor passes high frequencies more and more easily. This is why capacitors are used to block DC while coupling AC signals, and why a capacitor forms the frequency-selective part of RC filters and LC tuned circuits.
How do I calculate an RC filter cutoff frequency?
The cutoff (or corner) frequency of an RC filter is fc = 1/(2πRC), where R is in ohms and C is in farads. It is the −3 dB point where the output has fallen to about 70.7% of the input and the capacitor's reactance equals the resistance. For example, a 1 kΩ resistor with a 100 nF capacitor gives fc = 1/(2π × 1000 × 100×10⁻⁹) ≈ 1.59 kHz. The same R and C make either a low-pass filter (output taken across the capacitor) or a high-pass filter (output across the resistor) — both share this cutoff frequency.
What is the resonant frequency of an LC circuit?
The resonant frequency of an LC circuit is f0 = 1/(2π√(LC)), where L is in henries and C is in farads. It is the frequency at which the inductive reactance XL = 2πfL and the capacitive reactance XC = 1/(2πfC) are equal and cancel, leaving the circuit purely resistive. For example, 1 mH with 1 µF resonates at 1/(2π√(10⁻³ × 10⁻⁶)) ≈ 5.03 kHz. A series LC is a near short circuit at f0 and a parallel LC is a near open circuit, which is how radios, oscillators and tuned filters select one frequency out of many.
What is the difference between reactance and resistance?
Both are measured in ohms and both oppose current, but resistance dissipates energy as heat and does not depend on frequency, while reactance stores and returns energy in a magnetic (inductor) or electric (capacitor) field and changes with frequency. Resistance keeps voltage and current in phase; reactance shifts them 90° apart. Combined, resistance R and net reactance X form the impedance Z = √(R² + X²), which is the total opposition an AC circuit presents at a given frequency.
Related electronics tools
Reactance is where the AC side of the electrical cluster begins: pair it with the 555 timer calculator to set oscillator frequencies, the capacitor code calculator to decode the C you need, Ohm's law for the resistive part of the circuit, and the resistor color code calculator to pick the R in an RC or RL filter.
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