RC Discharging Voltage at Time Calculator

Enter the initial voltage, resistance in ohms, capacitance in farads, and elapsed time in seconds. The result follows the standard exponential decay curve and also reports the RC time constant, decay factor, and percentage of the initial voltage remaining. It is useful for checking timing circuits, power-down behavior, sensor hold-up, and capacitor safety estimates without manually evaluating the exponential equation.

How the RC discharge equation describes voltage

A charged capacitor does not lose the same number of volts during every second. When it is connected across a resistor, its voltage follows the exponential relationship V(t) = V0 × exp(−t/(R × C)). V0 is the voltage at the instant discharge begins, t is elapsed time in seconds, R is resistance in ohms, and C is capacitance in farads. Their product, R × C, is the time constant τ in seconds. This calculator evaluates that equation directly and returns both the voltage and supporting values, making the result easier to inspect. At one time constant, the capacitor retains about 36.8 percent of its starting voltage. At two time constants it retains about 13.5 percent, and at five it retains less than one percent. The mathematical curve approaches zero continuously rather than reaching zero at a finite instant. That distinction matters when a circuit has a defined logic threshold, minimum operating voltage, or touch-safe target rather than an informal assumption that the capacitor is simply empty.

Enter units consistently and interpret the result

Use volts for initial voltage, ohms for resistance, farads for capacitance, and seconds for time. Convert common component markings before entering them: 10 kΩ is 10,000 ohms, 100 µF is 0.0001 farads, and 250 ms is 0.25 seconds. The calculator deliberately accepts plain numeric values instead of strings such as 10k or 100uF, which keeps unit assumptions explicit. Resistance and capacitance must both be greater than zero because a zero value would make the exponent undefined for this ideal discharge model. Time and initial voltage may be zero, but neither may be negative. The returned remaining voltage uses the same voltage scale as the initial value. The decay factor is a ratio from zero to one, while percent remaining expresses that same ratio from zero to 100. The time constant helps you check whether the elapsed time is short or long compared with the natural response of the circuit. Always compare calculated voltage with the actual threshold relevant to your design rather than relying only on a rounded number of time constants.

Understand the ideal model and practical limits

The calculation assumes an ideal capacitor with a constant capacitance discharging through one fixed resistance. Real circuits may behave differently because the load changes with voltage, a semiconductor path is nonlinear, capacitor leakage adds another discharge route, or equivalent series resistance creates a brief initial step. Component tolerances also affect the time constant: a resistor rated at ±5 percent and a capacitor rated at ±20 percent can produce a noticeably different voltage from the nominal calculation. Temperature, capacitor aging, dielectric absorption, and measurement equipment may add further variation. For design work, repeat the calculation with minimum and maximum plausible component values to estimate a useful envelope. For safety work, do not treat this ideal result as proof that stored energy is absent; measure the actual voltage and follow the equipment procedure. Within those limits, the equation is excellent for first-pass analysis, selecting a bleeder resistor, predicting a reset delay, or checking whether a hold-up capacitor remains above a device threshold. API requests are priced at $0.002, while the deterministic calculation itself requires no external data source.