This free RC filter calculator finds the cutoff frequency of a first-order RC low-pass or high-pass filter, or the resistor or capacitor you need for a target frequency. It also gives the RC time constant, the settling time and the gain in dB and phase shift at any frequency — useful for anti-aliasing filters, PWM smoothing, switch debouncing and AC coupling.
RC filter formulas
fc = 1 ÷ (2π R C) · C = 1 ÷ (2π R fc) · R = 1 ÷ (2π C fc)
τ = R × C
Low-pass gain: |H| = 1 ÷ √(1 + (f/fc)²), phase = −arctan(f/fc)
High-pass gain: |H| = (f/fc) ÷ √(1 + (f/fc)²), phase = +arctan(fc/f)
Worked example: anti-aliasing filter for an ADC
With R = 10 kΩ and C = 100 nF, fc = 1 ÷ (2π × 10,000 × 100 × 10⁻⁹) = 159 Hz. A 1 kHz noise signal is reduced to 1 ÷ √(1 + 6.28²) = 0.157, i.e. −16 dB. For a 1 kHz cutoff instead, keep R = 10 kΩ and use C = 1 ÷ (2π × 10,000 × 1000) ≈ 15.9 nF — the nearest standard value is 15 nF.
RC cutoff frequency chart
Cutoff (−3 dB) frequency for common resistor and capacitor values. It is the same for low-pass and high-pass filters.
| Capacitor | R = 1 kΩ | R = 10 kΩ | R = 100 kΩ |
|---|---|---|---|
| 1 nF | 159 kHz | 15.9 kHz | 1.59 kHz |
| 10 nF | 15.9 kHz | 1.59 kHz | 159 Hz |
| 100 nF | 1.59 kHz | 159 Hz | 15.9 Hz |
| 1 µF | 159 Hz | 15.9 Hz | 1.59 Hz |
| 10 µF | 15.9 Hz | 1.59 Hz | 159 mHz |
Where RC filters are used
- Anti-aliasing before an ADC, set well below half the sample rate.
- Debouncing switch and button inputs (often τ ≈ 5–20 ms).
- Smoothing PWM outputs into an analog voltage.
- AC coupling (high-pass) to block DC between amplifier stages.
- Power-on reset delays for logic and FPGAs.
Scaling a signal down before filtering it? Use the voltage divider calculator.