RC Time Constant Calculator
Enter a resistance and capacitance to find the RC circuit's time constant and approximate full charge time.
Resistance is in ohms, capacitance is in microfarads.
How the RC time constant works
The time constant (τ, tau) of a resistor-capacitor circuit is τ = R × C, where R is resistance in ohms and C is capacitance in farads. It represents the time needed for the capacitor to charge to about 63.2% of its final voltage, or discharge to about 36.8% of its starting voltage.
After 5 time constants (5τ), a capacitor is considered fully charged or discharged (over 99% complete) for practical purposes — a common rule of thumb used in timing circuit design.
Frequently asked questions
What does the time constant actually represent?
It's the time for a charging capacitor to reach about 63.2% of its final voltage, or for a discharging capacitor to drop to about 36.8% of its starting voltage. It's a single number that characterizes how fast the RC circuit responds.
Why 5 time constants for 'full' charge?
After 5τ, the capacitor has reached over 99% of its final charge — close enough to "fully charged" for most practical circuit design purposes, even though it mathematically never reaches exactly 100%.
What happens if I increase the resistance or capacitance?
Both increase the time constant proportionally — doubling either the resistance or the capacitance doubles τ, making the circuit charge or discharge more slowly.