Connect a charged capacitor to a resistor and it empties — not steadily, but fast at first and slower and slower after. The whole shape of that fall is governed by a single number.

$\tau = RC$, read aloud tau equals R C, with $\tau$ the Greek letter tau. $\tau$ is the time constant in seconds, $R$ is the resistance the charge must escape through, in ohms, and $C$ is the capacitance, in farads. Both factors multiply because both slow the discharge down: more resistance throttles the current, more capacitance means more charge to move. An ohm times a farad really is a second — that is worth checking through the units once, and then trusting for life.

The fall itself is $V = V_{0} \, e^{-t/\tau}$ — V equals V-nought e to the minus t over tau. $V_{0}$ (say V-nought) is the voltage at the instant the discharge starts, $V$ is the voltage after a time $t$ has elapsed, both in volts, and $t$ and $\tau$ are both in seconds so the exponent comes out bare — as any exponent must.

Two anchor facts are worth more than the algebra. After one τ, 37% is left (that is $1/e$, and it is where the time constant gets its meaning). After five τ, it is gone — 0.7% remains, which is why safety standards quote discharge times in multiples of five. The classic error is reading 37% as the amount GONE rather than the amount LEFT; that is the charging curve, and it is a different question.