RC / RL Filter Cutoff Calculator

This calculator finds the cutoff frequency of a simple RC or RL filter, the frequency that marks the boundary between the signals it passes and those it blocks. Filters are everywhere in electronics, used to remove noise, separate frequency bands, smooth power supplies, and shape audio and radio signals. The simplest filters are built from just a resistor and a capacitor (an RC filter) or a resistor and an inductor (an RL filter), and their defining characteristic is the cutoff frequency, also called the corner or minus 3 decibel frequency. At this frequency the filter's output has dropped to about 70 percent of the input in voltage, marking the transition between the passband and the stopband. Below or above it, depending on whether the filter is low-pass or high-pass, signals are increasingly attenuated. This tool computes it. You choose whether your filter is RC or RL, enter the resistance and either the capacitance or the inductance, and the calculator returns the cutoff frequency, along with the time constant, the angular frequency, and the period at cutoff. The results update as you type, so you can size components to land the cutoff where you want it. Use it to design filters for audio, power supplies, sensors or radio, to choose component values, or for electronics study. The relationships are neat: for an RC filter the cutoff is one over two pi times resistance times capacitance, and for an RL filter it is the resistance over two pi times the inductance. The time constant, resistance times capacitance for RC or inductance over resistance for RL, sets how quickly the filter responds, and the cutoff frequency is simply one over two pi times that time constant.

RC: fc = 1 / (2 pi R C). RL: fc = R / (2 pi L). The cutoff is the -3dB point, ~70.7% of input voltage. Use scientific notation for small C (e.g. 1e-6).