RC Time Constant Calculator
Find the RC time constant, the 63% charge time, and the full-charge time from resistance and capacitance.
How it works
When a capacitor charges through a resistor, it doesn't fill instantly — it eases toward the supply voltage on a curve. The time constant τ = R × C sets how fast that curve moves.
One time constant is the moment the capacitor reaches about 63% of the way to full. With a 10 kΩ resistor and a 100 µF capacitor, τ is 10000 × 0.0001 = 1 second, so it hits 63% after one second.
The charge never quite reaches 100%, but after five time constants it's within a fraction of a percent — close enough to call it full. That 5τ figure is the rule of thumb engineers use for timing and settling in RC circuits.
Frequently asked questions
Why 63% and not a rounder number?
It comes straight from the math: after one time constant the capacitor reaches 1 − 1/e of full charge, and 1/e is about 0.368, leaving roughly 63.2%. The odd number is a natural consequence of exponential charging.
Does the same time constant apply to discharge?
Yes. Discharging follows the same curve in reverse — after one τ the capacitor has dropped to about 37% of its starting voltage, and after 5τ it's essentially empty.
Where do RC time constants show up?
Everywhere timing or smoothing is needed: blinker circuits, debounce filters on buttons, low-pass filters that remove noise, and the reset delays that hold a chip steady at power-on.