RC Time Constant

The RC time constant, τ, is resistance times capacitance. It is the time a capacitor takes to charge to 63.2% of the supply voltage, or discharge to 36.8% of its starting voltage, through a resistor.

A 10 kΩ resistor and a 100 µF capacitor give τ = 1 second. The capacitor reaches 86.5% at 2τ, 95.0% at 3τ, 98.2% at 4τ and 99.3% at 5τ, which counts as fully charged. Discharging to half voltage takes τ x ln 2, or 0.69 seconds.

The capacitor charging time calculator and capacitor discharge time calculator use τ for every result. The guide to the capacitor time constant has the full table.