Find the −3 dB cutoff frequency and time constant of a first-order
RC or RL filter — or work backwards from a target cutoff to the component
you need. Everything runs in this page; nothing is uploaded.
Filter
The solved component is snapped to the nearest standard preferred value you can actually buy, and the cutoff that value really produces is shown.
Common questions
How do I calculate the cutoff frequency of an RC filter?
For a first-order RC filter the -3 dB cutoff frequency is f_c = 1 / (2·π·R·C), where R is in ohms and C is in farads. The time constant is τ = R·C. Whether the filter is low-pass or high-pass only changes where the output is taken, not the cutoff frequency.
What is the cutoff frequency of an RL filter?
For a first-order RL filter the -3 dB cutoff frequency is f_c = R / (2·π·L), with R in ohms and L in henries. The time constant is τ = L / R.
What does the -3 dB cutoff frequency mean?
It is the frequency at which the filter's output power has fallen to half of the input (voltage down to about 70.7%). Beyond it, a first-order filter rolls off at -20 dB per decade (-6 dB per octave).