RC Charge Percentage Calculator
A capacitor charging through a resistor follows an exponential curve set by the product of the resistance and the capacitance, called the time constant. The capacitor reaches about 63 percent of the supply voltage in one time constant and is practically full after five. This calculator takes the resistance, the capacitance, the supply voltage, and an elapsed time, then returns the time constant, the percentage charged, the voltage reached, and how many time constants have passed so you can read the charging curve precisely.
RC charging formula
Time constant tau = R * C
Fraction charged = 1 - e^(-t / tau)
Percentage charged = 100 * fraction
Voltage = supply * fraction
Time constants elapsed = t / tau
The exponential approach means each additional time constant closes about 63 percent of the remaining gap to the supply voltage. The curve never reaches exactly 100 percent but is within 1 percent after about five time constants.
RC charging facts
- After 1 time constant the capacitor is about 63.2 percent charged.
- After 2 time constants it is about 86.5 percent charged.
- After 3 time constants it is about 95.0 percent charged.
- After 5 time constants it is about 99.3 percent charged.
- A discharging capacitor falls to about 36.8 percent after one time constant.
RC charge percentage: frequently asked questions
What is the RC time constant?
The time constant tau equals the resistance times the capacitance. It is the time for a charging capacitor to reach about 63.2 percent of its final voltage, or for a discharging one to fall to about 36.8 percent. After five time constants the capacitor is considered fully charged or discharged for practical purposes.
How is the charge percentage calculated?
For a capacitor charging through a resistor toward a supply voltage, the fraction charged after time t is 1 minus e to the power of minus t over tau, where tau is resistance times capacitance. Multiplying by 100 gives the percentage of the final voltage reached.
Why is the capacitor 63 percent charged after one time constant?
The charging follows an exponential curve. After one time constant the exponent is minus 1, so e to the minus 1 is about 0.368, and 1 minus 0.368 is about 0.632. That is the 63.2 percent figure. The curve is steep at first and flattens as it approaches the supply voltage.
When is a capacitor considered fully charged?
Mathematically a capacitor never reaches exactly 100 percent, but after five time constants it is at about 99.3 percent, which is treated as fully charged in most engineering work. This calculator reports the exact percentage so you can judge it against your own tolerance.
Does this apply to discharging too?
Yes, by symmetry. A discharging capacitor's voltage is the starting voltage times e to the minus t over tau, so the percentage remaining is 100 minus the charge percentage shown here. After one time constant about 36.8 percent of the initial voltage remains.
Official sources
- NIST: Physical Measurement Laboratory: capacitance and resistance units.
- IEEE: IEEE standards for circuit terminology.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.