Resonant Frequency Calculator (LC)
The frequency at which an inductor and capacitor resonate, from their values. Used for tuned circuits, oscillators, filters and antenna matching.
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The formula
f₀ = 1 ÷ (2π√(LC))
Resonance is where the two reactances are equal and cancel. Inductive reactance rises with frequency and capacitive reactance falls, so they cross at exactly one point — and that crossing is the resonant frequency. Resistance does not appear, because it does not move the crossing point.
What to watch out for
Worth knowing about the result:
- Resistance does not change it. It changes how sharp the resonance is, but not where it sits. The RLC calculator covers that.
- It moves as the square root. To halve the resonant frequency you have to quadruple L or C, which is why tuning ranges need large variable capacitors.
- Stray values count at RF. Above a few MHz, the capacitance of the board and the inductance of the leads are part of the circuit whether you intended them or not.
Frequently Asked Questions
How do you calculate resonant frequency?
f₀ = 1 ÷ (2π√(LC)), with L in henries and C in farads. A 100 µH inductor with a 100 pF capacitor resonates at about 1.59 MHz.
What happens at resonance?
The inductive and capacitive reactances are equal and cancel, leaving only resistance. A series LC becomes a low impedance at that frequency; a parallel LC becomes a high one. That is what makes it a tuned circuit.
Does resistance affect resonant frequency?
Not in this formula. Resistance sets the Q factor — how sharply the circuit selects that frequency — but the resonant point itself depends only on L and C.