LC Filter Cutoff Frequency Calculator
An LC filter combines an inductor and a capacitor to give a second-order frequency response with a steeper 40 dB per decade rolloff than a single RC stage. Its cutoff sits at the LC resonant frequency, set entirely by the inductance and capacitance. This calculator returns the cutoff frequency, the angular resonant frequency, and convenient kilohertz and megahertz forms from L and C. LC filters are the backbone of radio front ends, switching power supply output stages, and any design needing a sharp frequency corner. Enter L in henries and C in farads.
LC filter cutoff formula
cutoff frequency f = 1 / (2 * pi * sqrt(L * C))
angular resonant frequency omega = 1 / sqrt(L * C)
relation omega = 2 * pi * f
second-order rolloff = 40 dB per decade
L is in henries, C in farads. The cutoff coincides with the LC resonant frequency. The actual sharpness of the response also depends on circuit and load resistance.
Filter notes
- An LC filter rolls off at 40 dB per decade, twice as steep as a single RC stage.
- 1 microhenry equals 0.000001 henry; 1 millihenry equals 0.001 henry.
- 1 nanofarad equals 0.000000001 farad; 1 microfarad equals 0.000001 farad.
- The cutoff depends only on L and C, not on which is low or high pass.
- Resistance and load damp the resonant peak; ideal LC has an infinitely sharp peak.
LC filter cutoff: frequently asked questions
What is the LC filter cutoff formula?
The resonant (cutoff) frequency of an LC filter is f = 1 / (2 * pi * sqrt(L * C)), with L in henries and C in farads, giving f in hertz. This is the same expression as the LC resonant frequency, since a second-order LC filter's corner sits at resonance.
How does an LC filter differ from an RC filter?
An LC filter uses an inductor and a capacitor and gives a second-order, 40 dB per decade rolloff, twice as steep as a single-pole RC filter. It also has a resonant peak whose sharpness depends on circuit resistance and load, unlike the gentle RC response.
What is the angular resonant frequency?
The angular resonant frequency is omega = 1 / sqrt(L * C) in radians per second. It relates to the ordinary frequency by omega = 2 * pi * f. Both describe the same resonance, one in hertz and one in radians per second.
What units should I enter?
Enter inductance in henries and capacitance in farads. For practical parts, convert first: 1 millihenry is 0.001 henry, 1 microhenry is 0.000001 henry, and 1 microfarad is 0.000001 farad. The formula needs base SI units to return hertz.
Does the rolloff depend on the filter arrangement?
The cutoff frequency depends only on L and C, but whether the filter is low pass or high pass, and how sharp the peak is, depends on the topology and any series or load resistance. This calculator gives the resonant corner frequency common to both arrangements.
Official sources
- NIST: SI units (hertz, henry, farad).
- NASA Glenn Research Center: LC resonance and filter fundamentals.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.