RC Time Constant Calculator
Calculate τ (tau) and visualize charge/discharge curves
Time Constant (τ)
τ = R × C
Voltage vs Time
Time to Reach Target Voltage
Charging
V(t) = V₀ × (1 - e-t/τ)
Capacitor charges toward supply voltage. Reaches 63.2% at t = τ.
Discharging
V(t) = V₀ × e-t/τ
Capacitor discharges toward 0V. Falls to 36.8% at t = τ.
Time Constant
τ = R × C
The time for voltage to reach 63.2% (charge) or fall to 36.8% (discharge).
Time to Voltage
t = -τ × ln(1 - V/V₀)
Calculate time to reach any target voltage while charging.
Understanding RC Circuits
An RC circuit consists of a resistor and capacitor. When voltage is applied, the capacitor charges through the resistor. The time constant τ (tau) determines how fast this happens.
Key Points
- 1τ: 63.2% charged / 36.8% remaining
- 2τ: 86.5% charged / 13.5% remaining
- 3τ: 95.0% charged / 5.0% remaining
- 4τ: 98.2% charged / 1.8% remaining
- 5τ: 99.3% charged — considered "fully" charged
Applications
RC circuits are used for timing (like the 555 timer), filtering (low-pass/high-pass), debouncing switches, power supply smoothing, and analog-to-digital conversion.