RC Filter Cutoff Frequency Calculator

Start with the required cutoff frequency and then choose the resistor value first. Pick R large enough that it does not significantly load the source (typically R ≥ 1 kΩ, or at least 10 times the source impedance), but small enough that the required capacitor value is physically achievable and not excessively affected by capacitor tolerances. From fc and R, calculate C = 1/(2π⋅fc⋅R). For example, a 1 kHz cutoff with R = 10 kΩ gives C = 1/(2π⋅1000⋅10000) = 15.9 nF, which you would round to 15 nF or 22 nF from standard values. Always use a capacitor tolerance of at least 5% for filter circuits; standard ceramic capacitors are 10-20% tolerance, so the actual cutoff frequency will vary accordingly.

Two loading effects are the most common culprits. Source impedance in series with R raises the effective resistance seen by C, shifting the cutoff frequency lower than calculated and reducing attenuation at the target frequency. Load impedance in parallel with C (the input impedance of the next stage) lowers the effective capacitance, shifting the cutoff higher. The solution is to buffer both sides of the filter with op-amp stages, or to design the R value to be much smaller than the load impedance and much larger than the source impedance. In practical circuits, keeping source impedance less than R/10 and load impedance greater than 10⋅R keeps the actual response within a few percent of the calculated ideal.