RC Filter & Time Constant Calculator
Cutoff frequency, time constant and rise time for an RC pair.
How it works
The time constant is τ = R x C — the time to reach 63.2% of a step change. Five time constants gets you to 99.3%, which is what "settled" means in practice.
The cutoff frequency is where the same circuit attenuates by 3 dB:
fc = 1 / (2πRC)
At that frequency the capacitor's reactance equals the resistance exactly, and the output lags the input by 45 degrees. Whether the circuit is a low-pass or a high-pass depends only on which component you take the output across, not on the values.
Common questions
Why 63.2%?
It is 1 - 1/e. The charging curve is exponential, and one time constant is the point where the exponent reaches -1.
How do I get a steeper roll-off?
Cascade stages or use an active filter. A single RC rolls off at 6 dB per octave and no choice of R and C changes that — only the frequency at which it starts.