Capacitor Charge Time Calculator
When a capacitor charges through a resistor, its voltage rises along a smooth exponential curve set by the RC time constant. This behaviour governs timing circuits, power-up sequencing, filter settling, and debounce delays. The time constant is simply the resistance times the capacitance, and after about five time constants the capacitor is effectively fully charged. Enter the resistance, the capacitance, the supply voltage, and a target voltage; this calculator returns the time constant, the practical full-charge time, the voltage reached after one time constant, and the time to reach your target.
RC charging formula
Time constant = R * C
Voltage at time t = Vsupply * (1 - e^(-t / tau))
Voltage after 1 tau = Vsupply * 0.632
Full charge time = 5 * tau
Time to target = -tau * ln(1 - Vtarget / Vsupply)
R is in ohms and C in farads, giving the time constant in seconds. The target time is only defined when the target voltage is below the supply voltage.
Capacitor charging context
- The time constant tau equals R times C and sets the whole charging timescale.
- After one tau the capacitor reaches 63.2 percent, after three tau about 95 percent, and after five tau over 99 percent of the supply.
- Larger resistance or capacitance lengthens the charge time proportionally.
- A capacitor cannot charge above the supply voltage and approaches it asymptotically.
- The same time constant governs discharge, where the voltage decays rather than rises.
Capacitor charge time: frequently asked questions
What is the RC time constant?
The time constant of a resistor-capacitor circuit, written tau, equals the resistance times the capacitance (tau = R x C). After one time constant a charging capacitor reaches about 63.2 percent of the supply voltage, and after five time constants it is about 99.3 percent charged.
How long does it take to fully charge a capacitor?
A capacitor never charges to exactly the supply voltage in theory, but it is considered fully charged after about five time constants, when it has reached more than 99 percent of the supply. This calculator reports the five-tau time as the practical full-charge time.
What is the charging voltage equation?
The voltage across a charging capacitor is V(t) = Vsupply x (1 - e^(-t / tau)), where e is Euler's number and tau is the time constant. The voltage rises quickly at first and then levels off as it approaches the supply.
How do I find the time to reach a target voltage?
Rearranging the charging equation, the time equals minus tau times the natural logarithm of one minus the target voltage divided by the supply voltage. This calculator returns that time for the target voltage you enter, provided the target is below the supply.
Does this apply to discharging too?
The same time constant governs discharging, but the voltage falls as Vsupply x e^(-t / tau) instead of rising. After one time constant a discharging capacitor falls to about 36.8 percent of its starting voltage. This calculator models charging from zero.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.