Low Pass Filter Calculator
A low-pass filter passes signals below its cutoff frequency and attenuates signals above it. This calculator handles both RC and RL passive low-pass filters. Enter your component values to find the cutoff frequency, or specify a target frequency to calculate the required capacitor or inductor value.
The cutoff frequency formulas for first-order passive low-pass filters:
- RC filter: fc = 1 / (2piRC)
- RL filter: fc = R / (2piL)
Both are first-order filters with a -20 dB/decade (or -6 dB/octave) rolloff above the cutoff frequency. At the cutoff frequency itself, the output is -3 dB (70.7% of input voltage).
Choosing between RC and RL:
- RC filters are by far the most common for low-power signal filtering. Capacitors are cheap, small, and available in wide value ranges
- RL filters are used in power applications where inductors handle high currents better, and at frequencies where practical inductor values are small
For steeper rolloff, consider:
- Second-order (LC) filter: -40 dB/decade rolloff using both an inductor and capacitor
- Active filters: Op-amp-based Butterworth, Chebyshev, or Bessel filters can achieve any order without inductors
- Cascaded RC stages: Multiple RC stages separated by buffers, each adding -20 dB/decade