An LC circuit — a coil (inductor) and capacitor wired together — is one of the most fundamental building blocks in electronics, forming the resonant "tank" at the heart of radio tuners, oscillators, and filters. This calculator finds the resonant frequency of an LC circuit, or solves backward for the inductance or capacitance needed to hit a target frequency, using the standard formula f = 1/(2π√(LC)).
How resonant frequency works
In an LC circuit, the inductor opposes changes in current while the capacitor opposes changes in voltage. When energized, the circuit oscillates: energy shuttles back and forth between the inductor's magnetic field and the capacitor's electric field. At the resonant frequency f = 1/(2π√(LC)), the inductive reactance (XL = 2πfL) exactly equals the capacitive reactance (XC = 1/(2πfC)), so the two cancel and the circuit oscillates most freely at that single frequency.
This is why LC circuits are called "tank" circuits — like a tank sloshing water back and forth, energy sloshes between the two components. Real-world tank circuits also have some resistance, which damps the oscillation over time (see an RC or RLC analysis for that behavior) — this calculator models the ideal, lossless LC resonance used for initial component selection.
Inputs and what they mean
Inductance (L) is entered in henries, millihenries, or microhenries — most practical coils fall in the µH to mH range. Capacitance (C) is entered in farads, microfarads, nanofarads, or picofarads — tuning and RF capacitors are commonly nF or pF. Both L and C affect frequency by a square-root relationship, so doubling either one lowers the resonant frequency by a factor of √2, not by half.
On the Solve L and Solve C tabs, enter the frequency you're targeting (Hz, kHz, or MHz) alongside whichever component value is already fixed in your design, and the calculator returns the component value needed to complete the circuit.
Limits and edge cases
This calculator assumes an ideal LC circuit with no resistance. Real circuits include parasitic resistance in the coil windings and dielectric losses in the capacitor, which introduce damping and a finite bandwidth (quality factor, or Q) around the resonant peak — not modeled here. It also assumes lumped-element behavior; at very high frequencies, parasitic capacitance and inductance in the physical layout can shift the effective resonance away from the calculated value. For circuits with meaningful series resistance, use an RLC or RC time-constant analysis alongside this tool.