RL Low-Pass Cutoff

How to use this calculator

An RL low‑pass filter uses a series inductor with a shunt resistor; the output is taken across the resistor. Its cutoff frequency is fc = R / (2πL), with time constant τ = L/R. Enter L and R in any units and the −3 dB point appears instantly, plus the angular cutoff ωc = R/L.

What it means in a real circuit

Like the RC low‑pass, it rolls off at −20 dB/decade above fc, but the inductor's DC resistance and core losses make the real response less ideal at high Q. RL filters appear in power‑stage smoothing, RF chokes, and current‑sensing paths where an inductor is already present. It's the dual of the RC low‑pass: swap R↔L and C↔(1/R) and the math mirrors.