How long does a capacitor take to discharge?

About 5 time constants to fall below 1% of its starting voltage. After 5τ only 0.7% remains. With a 10 kΩ resistor and a 100 µF capacitor, τ is 1 second, so it is nearly empty after 5 seconds. Reaching half voltage takes RC x ln 2, which is 0.69 seconds.

The voltage falls as V(t) = V0 x e^(-t/τ). After one time constant 36.8% remains, then 13.5%, 5.0%, 1.8% and 0.7%. To find the time to any voltage, use t = τ x ln(V0 / Vtarget).

Use the resistance of the actual discharge path, which may differ from the charging resistor. A capacitor with no discharge path can hold its charge, so treat large capacitors with care. The capacitor discharge time calculator runs these numbers, and the guide to the capacitor time constant shows the full curve.