TFT

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

1

Enter resistance value

Input the resistance in ohms (Ω). This is the resistor in your RC circuit.

2

Enter capacitance value

Input the capacitance in farads (F). For typical circuits, this will be in µF or pF.

3

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.

τ = R × C

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

TimeCharge LevelVoltage (% of max)
1τ63.2%0.632 × Vmax
2τ86.5%0.865 × Vmax
3τ95.0%0.950 × Vmax
4τ98.2%0.982 × Vmax
5τ99.3%~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.

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