Enter the resistance and capacitance of a simple charging circuit to get the time constant and a full charge progression. Add the supply voltage to also see the voltage reached at each step.
Leave this blank to see the charge progression as a percentage only. Fill it in to also see the voltage reached at each step.
Charge progression by time constant:
| Elapsed time | Charge reached | Time | Voltage reached |
|---|
This calculation assumes an ideal capacitor starting fully discharged, a fixed resistance, and no leakage current. Real electrolytic capacitors typically carry a tolerance of about +-20%, so treat these figures as a close estimate rather than an exact reading.
Why a capacitor never quite reaches 100%
When a capacitor charges through a resistor from a fixed DC supply, its voltage follows V(t) = Vmax x (1 - e^(-t / RC)), a curve that keeps flattening out but mathematically never touches the supply voltage exactly. The product R x C, called the time constant (τ), sets the pace: after one τ the capacitor holds about 63% of the supply voltage, and after five τ it is close enough to full (about 99%) to treat as charged in practice. This matters on a UK 230V mains-adjacent DC rail just as much as on a 9V PP3 battery circuit, since the percentage reached at each τ never changes, only the time it takes and the final voltage do.
The charge progression by time constant
| Elapsed time | Charge reached |
|---|---|
| 1τ | 63.21% |
| 2τ | 86.47% |
| 3τ | 95.02% |
| 4τ | 98.17% |
| 5τ | 99.33% |
What this calculator cannot do
The formula above assumes an ideal resistor and capacitor fed from a source with no internal resistance, and it ignores the leakage current that lets a real capacitor slowly lose charge over time even with no load attached. UK-sourced electrolytic capacitors commonly carry a tolerance of about +-20% on their printed value under BS/EN 60384, so a physical circuit will charge slightly faster or slower than this tool predicts. Discharge time through a resistor follows the mirror curve, V(t) = Vmax x e^(-t / RC), and reaches the same time constant milestones in reverse.
A worked example
A 10kΩ resistor in series with a 100µF capacitor gives τ = 10,000 x 0.0001 = 1 second, so the capacitor reaches about 63% of the supply voltage after 1 second and about 99% after 5 seconds, whatever the supply voltage happens to be. This RC delay is the same principle behind a camera flash charging between shots or a simple power-on reset circuit on a microcontroller board. For the resistor side of the same circuit, the Ohm's Law Calculator covers voltage, current, resistance, and power together.