LC Filter Cutoff Frequency Calculator

Passive filter cutoff

First-order RC/RL/LC filters roll off at −20 dB/decade after f_c: f_c(LC) = 1/(2π√(LC)) (undamped), f_c(RC) = 1/(2πRC), f_c(RL) = R/(2πL). Choose 2nd order for a −40 dB/decade skirt and set Q: 0.707 (Butterworth, maximally flat), 0.5 (Bessel, linear phase), or higher for peaking near f_c.

At f_c the output is −3 dB for first order (and Butterworth 2nd order). Use LC for power filtering (efficiency) and RC where cost matters.