Capacitor Discharge Time Calculator
A capacitor discharging through a resistor loses voltage along an exponential curve set by the RC time constant. This calculator finds how long it takes the capacitor to fall from a starting voltage to a chosen target voltage, given the resistance and capacitance. It also reports the time constant tau and the five-tau time that engineers treat as effectively fully discharged. Use it for safety bleed-down times on power supplies, timing circuits, and any design where you need to know when stored charge has drained to a safe or usable level.
Capacitor discharge formula
time constant tau = R * C
voltage decay V(t) = V0 * e^(-t / tau)
discharge time t = R * C * ln(V0 / V)
effectively discharged at t = 5 * tau
R is in ohms, C in farads, giving tau and t in seconds. The natural logarithm of the voltage ratio scales the time constant to reach the target.
Practical notes
- After one tau the voltage is about 36.8 percent of the start; after five tau, under 1 percent.
- 1 microfarad equals 0.000001 farad; 1 kilohm equals 1,000 ohms. Convert before entering.
- Only the ratio V0 / V matters, so consistent voltage units suffice.
- The target voltage must be greater than zero and less than the start voltage.
- Real circuits have leakage and series resistance that can change the actual decay.
Capacitor discharge: frequently asked questions
What is the RC time constant?
The time constant tau equals resistance times capacitance, tau = R * C, measured in seconds when R is in ohms and C is in farads. In one time constant a discharging capacitor falls to about 36.8 percent of its starting voltage; after five time constants it is below 1 percent and considered fully discharged.
How do you calculate capacitor discharge time?
A capacitor discharging through a resistor follows V(t) = V0 * e^(-t / (R*C)). Solving for time gives t = R * C * ln(V0 / V), where V0 is the starting voltage and V is the target voltage. The result is the time to fall from V0 down to V.
How long until a capacitor is fully discharged?
There is no exact zero in an ideal exponential decay, but five time constants (5 * R * C) leaves under 1 percent of the initial voltage and is the standard engineering rule for a capacitor being effectively discharged.
What units should I use?
Enter resistance in ohms and capacitance in farads for the time constant in seconds. For practical components, convert first: 1 microfarad is 0.000001 farad and 1 kilohm is 1,000 ohms. Voltages can be in any consistent unit since only their ratio matters.
Why must the target voltage be below the start voltage?
Discharge only reduces voltage, so the target must be lower than the starting voltage and greater than zero. If the target is at or above the start, the logarithm is zero or undefined and no positive discharge time exists; the calculator returns n/a.
Official sources
- NIST: SI units (ohm, farad, second, volt).
- NASA Glenn Research Center: RC circuit fundamentals reference.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.