Pi Filter Attenuation Calculator

About this calculator

A pi filter (named for its schematic resemblance to the Greek letter π) is an LC low-pass filter — typically capacitor, inductor, capacitor — used to smooth ripple out of a rectified power supply or to reject unwanted frequencies in RF circuits. Because it combines an inductor and capacitor, it’s a second-order (two-pole) filter, and beyond its cutoff frequency f_c = 1/(2π√(LC)) its attenuation rolls off at a standard 40 dB per decade (equivalently 12 dB per octave) — twice as steep as a single-pole RC filter.

This calculator takes the inductance, capacitance, and the ripple frequency you need to suppress, and estimates how many decibels of attenuation the pi filter provides at that frequency using the two-pole rolloff. Power-supply designers use exactly this kind of estimate to size L and C so that mains-frequency ripple (100/120 Hz after rectification) is knocked down to an acceptable residual level, and RF engineers use the same topology to filter unwanted harmonics from a signal path.