RC Time Constant Calculator

In short The RC time constant is simply resistance times capacitance, and with consistent units it comes out in seconds: 1 kΩ with 1 µF gives exactly 1 ms. What that number means is the useful part. After one time constant the capacitor has reached 63.2% of the supply, and 63.2% is not an arbitrary figure — it is 1 − 1/e. The capacitor charges at a rate proportional to how far it still has to go, so each time constant covers the same fraction of whatever remains, which is why it never quite arrives. By convention a capacitor is considered charged after five time constants, at 99.3%. Mathematically it is still climbing; practically nothing can measure the difference. Discharging is the mirror image: 36.8% left after one time constant, 0.7% after five. The same RC network is also a first-order filter, with its corner at 1 ÷ (2πRC) — 159.15 Hz for that 1 kΩ and 1 µF — where the output is 3 dB down. Take the output across the capacitor and it is a low-pass; swap the two components and it is a high-pass with the same corner. And for debouncing a mechanical switch, aim for a time constant of a few milliseconds: bounce usually settles within 5 to 20 ms, so much shorter lets it through and much longer makes the button feel sluggish.