RC Time Constant

The RC time constant (τ = tau) defines how fast a capacitor charges or discharges through a resistor. After one time constant (1τ), the capacitor reaches 63.2% of the supply voltage during charging. After five time constants (5τ), it reaches 99.3% and is considered fully charged. This is fundamental to filter design, timing circuits, and signal processing.

Resistance (kΩ)

Capacitance (µF)

Supply voltage (V)

Elapsed time (s)

RC network

Time constant τ1 s

Full charge (~5τ)5 s

VC at t7.585 V (63.2%)

Cut-off frequency159.2 mHz

Formulas used

τ = R × C · Full Charge ≈ 5 × τ · V(t) = Vs × (1 − e−t/τ)

How to use this calculator

Step-by-Step Instructions

Enter resistance in Ohms The resistor through which the capacitor charges or discharges.
Enter capacitance in µF The capacitor value. Convert nF to µF by dividing by 1000 if needed.
Click Calculate Results show the time constant (τ) and the voltage at 1τ through 5τ intervals.

Worked Example

R = 10kΩ (10000Ω), C = 100µF τ = 10000 × 0.0001 = 1.0 second

Full charge time (5τ) = 5.0 seconds