RC Time Constant Calculator – Calculate RC Circuit Time Constant
Calculate the time constant for RC circuits. τ = R × C determines charging and discharging rates.
How to Use This RC Time Constant Calculator
Enter resistance value
Input the resistance in ohms (Ω). This is the resistor in your RC circuit.
Enter capacitance value
Input the capacitance in farads (F). For typical circuits, this will be in µF or pF.
View time constant results
The calculator shows τ (tau) and the full charge time (5τ) for your circuit.
Understanding RC Time Constant
The RC time constant (τ, tau) determines how quickly a capacitor charges or discharges through a resistor. It is the product of resistance and capacitance.
After one time constant, the capacitor reaches 63.2% of its final voltage. After 5τ, it is 99.3% charged — considered fully charged for most purposes.
Capacitor Charging Progress
| Time | Charge Level | Voltage (% of max) |
|---|---|---|
| 1τ | 63.2% | 0.632 × Vmax |
| 2τ | 86.5% | 0.865 × Vmax |
| 3τ | 95.0% | 0.950 × Vmax |
| 4τ | 98.2% | 0.982 × Vmax |
| 5τ | 99.3% | ~Fully charged |
The same timing applies to discharging, but the percentages show remaining charge.
Common RC Circuit Applications
Timing Circuits
Generate precise delays or time intervals in electronic circuits
Filter Design
Create low-pass or high-pass filters for audio and signal processing
Debouncing
Smooth out mechanical switch bounce in digital circuits
Power Supply Smoothing
Reduce ripple in rectified AC power supplies
Frequently Asked Questions
What is the RC time constant?
The RC time constant (τ) is the time it takes for a capacitor to charge to 63.2% of its final voltage through a resistor. It equals R × C, measured in seconds.
Why is 5τ considered full charge?
At 5τ, the capacitor reaches 99.3% of full charge. The remaining 0.7% takes disproportionately long to add. For practical purposes, 5τ is considered fully charged.
How do I convert µF to farads?
1 µF (microfarad) = 0.000001 F = 1×10⁻⁶ F. For example, 100 µF = 0.0001 F. Enter 0.0001 or 1e-4 in the calculator.
Does the time constant change with voltage?
No. The time constant depends only on R and C values, not the applied voltage. Higher voltage means higher final charge, but the timing stays the same.
What happens if I increase resistance?
Higher resistance means slower charging. Doubling R doubles the time constant. The capacitor takes twice as long to reach the same charge percentage.
Other Free Tools
Rl Time Constant Calculator
RL Time Constant Calculator – Calculate RL Circuit Time Constant
Rlc Resonance Calculator
RLC Resonance Calculator – Calculate Resonant Frequency
Inductor Calculations
Inductor Calculator – Inductance and Inductive Reactance Calculator
Series Parallel Capacitor Calculator
Capacitor Calculator – Series and Parallel Capacitance Calculator
Duty Cycle Calculator
Duty Cycle Calculator – Calculate PWM Duty Cycle
Pwm Frequency Calculator
PWM Frequency Calculator – Calculate PWM Output Frequency