Electrical · Analog circuits
RC Time Constant Calculator
Find τ from R and C, see cutoff frequency and 5τ settle time, and watch the series RC schematic highlight the value you are editing.
τ = R × C
Result
Time constant
τ = R × C
Enter R and C — τ and the schematic update as you type.
τ = R × C governs how fast a capacitor charges or discharges through a resistor.
The RC time constant τ (tau) is the time for the capacitor voltage to move by about 63.2% toward its final value when charging, or to fall to 36.8% of its initial value when discharging through the same resistor.
R in ohms, C in farads, τ in seconds. Five time constants (5τ) is the usual rule of thumb for “fully” charged or discharged (~99%).
Charging
| 0τ | 0% |
| 1τ | 63.2% |
| 2τ | 86.5% |
| 3τ | 95.0% |
| 5τ | 99.3% |
Discharging
| 0τ | 100% |
| 1τ | 36.8% |
| 2τ | 13.5% |
| 3τ | 5.0% |
| 5τ | 0.7% |
Time constant
τ = R × C
Cutoff frequency
fc = 1 / (2πτ)
Charging voltage
V(t) = Vf · (1 − e^(−t/τ))
Discharging voltage
V(t) = V0 · e^(−t/τ)
- Timing — monostables, delays, and pulse shaping
- Filters — first-order low-pass and high-pass corners
- Power — bulk capacitance and inrush time scales
- Coupling — AC blocks and DC restoration
- Debouncing — switch and encoder settling
- Component tolerances shift τ — use worst-case R and C for timing-critical paths.
- ESR and ESL on real capacitors matter at high speed; ideal τ is a first-order model.
- Load resistance in parallel with C changes the effective time constant.
- Initial voltage on C affects the curve but not τ itself.