This filters and resonance calculator finds the cutoff frequency of RC and RL filters and the resonant frequency of LC and RLC circuits, plus related figures like audio crossover points. It is essential for designing tone controls, power-supply filtering, radio tuning, and speaker crossovers.
Cutoff and resonant frequencies
A simple RC or RL low- or high-pass filter has a cutoff frequency where the signal is reduced to about 70% (−3 dB) of its passband level: f = 1 ÷ (2πRC) for RC. Below or above that point the filter increasingly attenuates. An LC or RLC circuit instead resonates at f = 1 ÷ (2π√(LC)), the frequency at which inductor and capacitor exchange energy most readily — the basis of radio tuning and notch filters. The calculator computes whichever you need from the component values.
These frequencies set where a circuit acts. A crossover splits audio between a woofer and tweeter at a chosen frequency; a power-supply filter removes ripple above its cutoff; a tuned circuit selects one station. Choosing R, L, and C to land the frequency where you want it is the heart of the design.
From frequency to components
Usually you know the frequency you want and need component values. The calculator rearranges the formulas so you can fix one component and solve for the other, then round to standard values. Remember real components have tolerance and parasitics, so verify a critical filter by measurement after building.
An RC low-pass filter with R = 1 kΩ and C = 0.1 µF.
- Cutoff f = 1 ÷ (2πRC).
- f = 1 ÷ (2π × 1000 × 0.0000001).
- f ≈ 1592 Hz.
The filter rolls off above about 1.6 kHz (its −3 dB cutoff frequency).
