A capacitor charging through a resistor never quite reaches full voltage – it gets 63.2% of the way in one time constant and 99.3% in five. Timers, debounce circuits and camera flashes rely on this predictable curve.
How to use the Capacitor Charge Time Calculator
- Resistance R
- Capacitance C
- Target
- Charge level (%)
- Number of time constants
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the resistance and capacitance, then the target charge percentage (or a number of time constants).
Inputs explained
| Field | What to enter |
|---|---|
| Resistance R | Units: kΩ, Ω, MΩ |
| Capacitance C | Units: µF, nF, pF, mF, F |
| Target | Choose one: Percentage of supply voltage, Multiple of τ |
| Charge level (%) | A number |
| Number of time constants | A number |
Example values
How the main result changes when Resistance R changes and everything else stays at the example values above:
| Resistance R (kΩ) | Charging time (s) |
|---|---|
| 5 | 2.4809 |
| 7.5 | 3.7214 |
| 10 | 4.9618 |
| 12.5 | 6.2023 |
| 15 | 7.4428 |
| 20 | 9.9237 |
Formula
τ = R·C, t = −τ·ln(1 − p/100), 5τ ≈ 99.3%
Worked example
Inputs — Resistance R: 10 kΩ; Capacitance C: 100 µF; Target: Percentage of supply voltage; Charge level (%): 99.3.
Results — Time constant τ = 1,000 ms; Charging time = 4.96185 s; Charge level reached = 99.3 %.
Key terms
- Time constant
- τ = RC.
Frequently asked questions
Why is 5τ considered fully charged?
The capacitor is at 99.3% of the supply.
Does discharging take as long?
Yes – the same exponential with the same τ.
What does τ mean physically?
The time to reach 63.2% of the final voltage.
How do I make a longer delay?
Increase R or C.