Capacitor Discharge Time Calculator — KiloWattCalculator.com
Electrical Calculators & Unit Converters
Solve for capacitor discharge time, resistance, capacitance, initial voltage, or target voltage — fill in any four values and leave the one you want blank. See the time constant, a discharge curve, and whether the target voltage is safe to touch.
Enter four of the five values below (initial voltage, target voltage, resistance, capacitance, time) and leave the value you want to solve for blank. All fields accept your preferred units.
A capacitor is considered fully discharged after 5 time constants (5τ), when less than 1% of the original voltage remains.
| Time Constants | Elapsed Time | Voltage Remaining | % Remaining |
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Results are estimates based on an ideal RC discharge model. Real-world capacitors, leakage paths, and dielectric absorption can affect actual discharge time.
A 100 microfarad capacitor and a 10 kilohm resistor have a time constant of one second, because tau is resistance times capacitance and 10,000 ohms times 0.0001 farad is 1. After one second, about 37 percent of the starting voltage is left. After five seconds, about 0.67 percent is left. The voltage never reaches a mathematical zero. Five time constants is the practical end of the curve, which is why the questions below call that point fully discharged for ordinary work.
Energy stored in a capacitor is one-half capacitance times voltage squared. The percent of voltage and the percent of energy are not the same number, because energy follows the square of the voltage. A large capacitor at a high starting voltage can still hold a meaningful amount of energy when the voltage looks "almost gone" on a percentage scale. The questions on this page already note that many safety references treat 50 volts as a common bare-hand threshold, and that the figure which governs a job is the standard or the manufacturer's instruction that applies to that equipment.
Leave resistance blank when the unknown is the bleeder, and enter the capacitance, the starting voltage, the target voltage, and the time. The time-constant table on the calculator stays closed until a result exists, because its elapsed-time column depends on that calculation. This guide does not replace that table. It is the reason the table is worth opening after Calculate.
The time constant, tau, equals resistance multiplied by capacitance (τ = R × C) and represents the time for a capacitor's voltage to fall to about 36.8% of its starting value. Discharge time for any target voltage is a multiple of tau, found with t = R × C × ln(V₀ / Vf).
A capacitor never mathematically reaches exactly zero volts — the voltage decays exponentially and only approaches zero. In practice, a capacitor is considered fully discharged after five time constants (5τ), at which point less than 1% (about 0.67%) of the original voltage remains.
Many general safety references treat 50 volts or lower as a common threshold before bare-hand contact, but the correct figure depends on the specific standard that applies — OSHA, UL, NEC, or the equipment manufacturer's documentation. Always confirm the applicable safe-touch voltage for your situation before handling a capacitor.
Solve the RC equation for resistance: R = t / (C × ln(V₀ / Vf)). Enter the capacitance, the starting voltage, the target safe voltage, and the discharge time you want, leaving resistance blank, and the calculator returns the resistor value needed.
As a capacitor discharges through a resistor, the discharge current is proportional to the remaining voltage (I = V / R), so the rate of voltage loss slows as the voltage drops. This self-limiting relationship produces the exponential decay curve V(t) = V₀ × e^(−t / RC) rather than a straight line.
Disclaimer: Results produced by this calculator are for reference and educational purposes only and are not a substitute for measuring an actual capacitor's voltage with a properly rated meter. Capacitors — especially large-value or high-voltage types — can retain a dangerous stored charge even when disconnected from a power source and even after passing the calculated discharge time. Always independently verify a capacitor's voltage with a meter before physical contact, and never rely on a calculated estimate alone. Safe-touch voltage thresholds vary by applicable standard and application; confirm the correct threshold for your situation. All values must be independently verified before use in any circuit design, repair, or component substitution, and any electrical work must comply with the National Electrical Code (NFPA 70) and the authority having jurisdiction and be performed or reviewed by a licensed electrician where required. KiloWattCalculator.com and EnterPlanet LLC accept no liability for decisions made on the basis of these calculations.