RC Time Constant Calculator
By Santaji GadepatilinUpdated October 27, 2026
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How to use this calculator
Enter the resistance and capacitance to get the time constant, approximate full-charge time and filter cut-off frequency. You can also solve for the capacitor needed for a target time.
Formulas
τ = R C t(≈99%) = 5τ fc = 1 ÷ (2π R C)
Worked example
10 kΩ and 100 µF:
- τ = 10,000 × 0.0001 = 1 s
- Full charge ≈ 5 s
- Cut-off frequency = 0.159 Hz
Charging curve
| Time | Charge (% of supply) |
|---|---|
| 1τ | 63.2% |
| 2τ | 86.5% |
| 3τ | 95.0% |
| 4τ | 98.2% |
| 5τ | 99.3% |
The same percentages apply in reverse when discharging. RC networks are used for timers, debouncing switches and simple signal filters.
Frequently asked questions
What is the RC time constant?
τ = R × C, the time for a capacitor to charge to about 63.2% of the supply voltage through a resistor.
How long does a capacitor take to charge fully?
About 5 time constants (5RC) to reach 99.3%.
What is the cut-off frequency of an RC filter?
fc = 1 ÷ (2πRC) — the frequency at which the output falls to 70.7% (−3 dB).