Charging Time Calculator
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Capacitor Charging Time Calculator

Work out how long a capacitor takes to charge through a resistor, from the RC time constant. Switch to constant current for a charger that holds the current steady.

Capacitor charging time

4.96 s

to 99.3% of 12 V

Time constant (R x C)
1 s
1 time constant
63.2%, 7.59 V
2 time constants
86.5%, 10.38 V
3 time constants
95%, 11.4 V
5 time constants
99.3%, 11.92 V
Time to 99.3%
4.96 s

Through a resistor, the voltage rises fast at first and then levels off. It never quite reaches 100%, so 5 time constants is treated as fully charged.

How long does a capacitor take to charge?

Through a resistor, a capacitor charges on a curve set by the time constant τ = R x C. A 10 kΩ resistor and a 100 µF capacitor give τ = 1 second. After one time constant the capacitor is at 63.2% of the supply voltage, and after five it is at 99.3%, which is treated as full. So that circuit is charged in about 5 seconds.

The voltage at any moment is V x (1 − e^(−t/RC)), the standard result for an RC circuit. The capacitor time constant guide shows where the 63% comes from.

τ = R x C time to a share p = −RC x ln(1 − p)

Capacitor charge by time constant

Share of the supply voltage after each multiple of RC.

Charging through a resistor
TimeChargeTime for 10 kΩ, 100 µF
1 τ63.2%1 s
2 τ86.5%2 s
3 τ95%3 s
4 τ98.2%4 s
5 τ99.3%5 s

Constant current charging

With a constant current source, the voltage rises in a straight line: t = C x ΔV / I. A 1 F capacitor charged to 5 V at 0.5 A takes 10 seconds. This is how supercapacitors are usually charged.

Frequently asked questions

How do you calculate capacitor charging time?

Multiply resistance in ohms by capacitance in farads to get the time constant in seconds. Five time constants charges the capacitor to 99.3%. 10,000 Ω x 0.0001 F = 1 s, so about 5 seconds to full.

Why is a capacitor fully charged after 5 time constants?

Each time constant closes 63.2% of the remaining gap. After five the capacitor is at 99.3% of the supply, close enough that circuits treat it as full. In theory it never quite reaches 100%.

What is the charging time of a capacitor with no resistor?

Without resistance the time constant would be zero, but every real circuit has some: wire, the supply's internal resistance and the capacitor's own ESR. That small resistance sets a very short charge time and a large inrush current.