LC Filter Calculator

An LC filter relies on the reactive properties of an inductor (L) and a capacitor (C) to block or pass specific frequency bands. At the core of this passive filter is the resonant or cutoff frequency equation: Fc = 1 / (2 * pi * sqrt(L * C)). In this formula, Fc represents the cutoff frequency in Hertz, L denotes inductance in Henries, and C represents capacitance in Farads. By rearranging this standard mathematical expression, our calculator allows you to solve for any unknown variable—whether you are looking for the exact frequency cutoff of existing components or determining what size capacitor or inductor you need to buy.

Let us walk through a practical engineering scenario: suppose you need to design an LC low-pass filter with a target cutoff frequency (Fc) of 1,000 Hz, and you have a readily available inductor (L) with a value of 10 millihenries (0.01 H). To find the required capacitance (C), we rearrange the formula to solve for C: C = 1 / ( (2 * pi * Fc)^2 * L ). First, calculate the angular frequency term: 2 * pi * 1000 = 6,283.18. Next, square this value to get 39,478,417. Multiply that result by the inductance (0.01), yielding 394,784.17. Finally, divide 1 by this product: C = 1 / 394,784.17, which results in approximately 2.53 microfarads (uF). Thus, pairing a 10 mH inductor with a 2.53 uF capacitor yields your desired 1 kHz cutoff.

When implementing physical LC filters, always account for component tolerances; standard off-the-shelf capacitors and inductors often deviate by 5% to 20% from their stated nominal values. Additionally, pay close attention to the parasitic resistance (ESR) of your inductors, as high resistance can damp the filter response and reduce overall efficiency. Finally, ensure your chosen components are rated to handle the maximum voltage and current passing through the circuit to prevent saturation or dielectric breakdown.