Low Pass Filter Cutoff Frequency Calculator

Real-world use cases

• Size an anti-aliasing filter before an ADC

  You sample a sensor at 1 kS/s and need to kill noise above the Nyquist frequency of 500 Hz. Drop in a passive RC low-pass with the corner around 200 Hz, pick a 10 kΩ resistor, and the design mode hands back C ≈ 80 nF. Round to a standard 82 nF and the corner lands close enough that aliasing is suppressed before the converter ever sees the signal.

• Design a speaker crossover point

  A two-way speaker needs the tweeter protected from bass. Set a 2.5 kHz high-pass corner for an 8 Ω tweeter, solve for the series capacitor, and read C ≈ 8 µF. Pair it with the matching low-pass inductor for the woofer and you have a clean first-order crossover, all from the same fc = 1/(2πRC) corner shared by both halves.

• Tune an LC filter for power-supply ripple

  A switching supply leaves 100 kHz ripple on the rail. Switch to LC mode, enter a 100 µH inductor and a 10 µF capacitor, and read the corner near 5 kHz. Because the LC roll-off is 40 dB per decade, that places the 100 kHz ripple more than 50 dB down, far quieter than a single RC stage could manage.

• Check a microcontroller debounce or sensor smoothing stage

  You added an RC to smooth a noisy analog line and want to confirm it is not eating your real signal. Enter the resistor and capacitor you actually placed, read the corner, and compare it to the bandwidth of the signal you care about. If the corner sits below your signal you know to shrink the capacitor before it rounds off real edges.

Common pitfalls

• Mixing up the capacitor unit. A 100 nF cap entered as 100 µF is off by a thousand, pushing the cutoff a thousand times too low. Always confirm pF / nF / µF before reading the corner — the prefix dropdown is there so you do not have to retype powers of ten.

• Assuming the high-pass has a different corner from the low-pass. They share the identical fc = 1/(2πRC). Switching topology does not move the corner; only the side that passes changes. Pick fc once, then decide low-pass or high-pass by where you tap the output.

• Treating the −3 dB point as a brick wall. The signal is already down 3 dB AT the corner and keeps rolling off gradually past it, not dropping to zero. A first-order filter only attenuates 20 dB per decade, so a frequency one decade past fc is still only 20 dB down.

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