Filter Cutoff Frequency Calculator
The cutoff frequency marks where a filter begins to attenuate a signal, the point at which the output power has dropped to half its passband value. For a simple first-order filter it is set by a single resistor paired with a capacitor or an inductor. Enter the resistance and either the capacitance or the inductance; this calculator returns the RC and RL cutoff frequencies in hertz, the angular cutoff frequency in radians per second, and the RC time constant. The cutoff is identical whether the stage is wired as low-pass or high-pass.
Cutoff frequency formula
RC cutoff = 1 / (2 * pi * R * C)
RL cutoff = R / (2 * pi * L)
Angular cutoff = 2 * pi * cutoff = 1 / (R * C)
RC time constant = R * C
R is in ohms, C in farads, and L in henries, giving the cutoff frequency in hertz. At the cutoff the output amplitude is about 70.7 percent of the input, the -3 dB point.
Filter design context
- The cutoff frequency is the -3 dB point, where output power falls to half the passband value.
- A first-order filter rolls off at 20 decibels per decade beyond the cutoff.
- The same cutoff applies whether the stage is wired as a low-pass or a high-pass filter.
- The angular cutoff in radians per second is two pi times the cutoff in hertz.
- For steeper roll-off, cascade stages or use higher-order topologies such as Butterworth or Chebyshev.
Filter cutoff frequency: frequently asked questions
What is the cutoff frequency of an RC filter?
The cutoff frequency of a first-order RC filter is one divided by two pi times the resistance times the capacitance: f = 1 / (2 x pi x R x C). At this frequency the output power is half the input power, a drop of 3 decibels.
What happens at the cutoff frequency?
At the cutoff frequency the filter's output amplitude falls to about 70.7 percent of the input, which is the -3 dB point where output power is halved. Beyond this point a first-order filter rolls off at 20 decibels per decade.
How is an RL filter cutoff calculated?
For a first-order RL filter the cutoff frequency is the resistance divided by two pi times the inductance: f = R / (2 x pi x L). This calculator computes both the RC and RL cutoff so you can compare the two topologies.
What is the angular cutoff frequency?
The angular cutoff frequency, in radians per second, is two pi times the ordinary cutoff frequency in hertz. For an RC filter it equals one over R times C. It is the natural frequency variable used in transfer-function analysis.
Does this work for low-pass and high-pass filters?
Yes. A simple RC or RL stage has the same cutoff frequency whether it is wired as a low-pass or a high-pass filter; only the position of the components and which node is the output change. The -3 dB cutoff point is identical.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.