What is τ (tau)?
The time constant τ = R × C tells you how fast a capacitor charges through a resistor. After one τ the capacitor reaches 63.2% of supply voltage. After 5τ it is considered fully charged at 99.3%.
Capacitor Charge / Discharge Calculator
V
τ
Charge Curve (0 – 5τ)
RC capacitor charge and discharge calculator with time constant
RC filter cutoff
Use f = 1/(2πRC) to find the −3 dB frequency. Signals below this pass through; signals above are attenuated. This applies to low-pass RC filters used in audio and sensor smoothing.
Design tip
For debouncing a switch or smoothing ADC readings, choose R and C so 5τ is much shorter than your sampling interval. For power-on-delay circuits, 5τ sets the activation time.