RC Filter Calculator (Cutoff Frequency)
Instantly compute the −3 dB cutoff frequency (fc) and time constant of an RC filter from one resistor and one capacitor.
Low-pass: passes signals below fc, attenuates above it (−3 dB at fc).
Disclaimer: This calculator is provided for general informational and educational purposes only, on an “as is” basis and without any warranty of accuracy or fitness for a particular purpose. Results may contain errors — always verify independently before relying on them in real designs. PartAndStock accepts no liability for any loss or damage arising from use of this tool, including when embedded on third-party sites.
How to use
- 1Pick the filter type: low-pass or high-pass.
- 2Enter the resistor in kΩ and the capacitor in nF.
- 3The cutoff frequency (fc) and time constant (τ) are computed instantly.
How it works
Enter a resistor and capacitor value to find the cutoff frequency (fc) and time constant (τ) of a low-pass or high-pass RC filter.
How an RC filter works
A resistor and a capacitor form a frequency-dependent voltage divider. Because the capacitor's reactance (Xc = 1/2πfC) changes with frequency, so does the filter's output. The cutoff frequency fc is where output power drops to half (−3 dB, ~70.7% amplitude), which happens exactly when the resistance equals the reactance: fc = 1/(2πRC).
Low-pass vs high-pass
Take the output across the capacitor for a low-pass filter: low frequencies pass, high ones are attenuated (noise filtering). Take it across the resistor for a high-pass: DC and low frequencies are blocked, high ones pass (capacitor coupling). Both use the same fc formula; a first-order filter rolls off at ±20 dB/decade.
Worked examples
- R=10 kΩ, C=100 nF → fc = 1/(2π·10k·100n) ≈ 159 Hz
- R=1 kΩ, C=10 nF → fc ≈ 15.9 kHz
- R=10 kΩ, C=100 nF → τ = RC = 1 ms
Common RC Combinations
| R | C | fc (approx.) |
|---|---|---|
| 1 kΩ | 100 nF | 1.59 kHz |
| 10 kΩ | 100 nF | 159 Hz |
| 10 kΩ | 10 nF | 1.59 kHz |
| 100 kΩ | 1 µF | 1.59 Hz |
| 1 kΩ | 10 nF | 15.9 kHz |