RC Time Constant Calculator

Timing an RC circuit

When a capacitor charges or discharges through a resistor, it does not do so instantly or at a constant rate - it follows an exponential curve, and the time constant (tau) describes how quickly that curve unfolds.

Worked example

For a 1000 ohm resistor and a 100uF capacitor:

tau = R x C = 1000 x 0.0001 = 100 ms

Frequently asked questions

What actually happens after one time constant? The capacitor reaches about 63.2% of its final charged voltage (or drops to 36.8% of its starting voltage during discharge) - it never mathematically reaches exactly 100%, which is why engineers use the "5 tau" rule of thumb (about 99.3% complete) to call a charge or discharge cycle practically finished.

Where does this show up in real circuits? RC time constants set the timing in debounce circuits, camera flash charge indicators, and the cutoff frequency of simple low-pass and high-pass filters.