RC Time Constant

How to use this calculator

Enter the resistance and capacitance; the time constant τ = R × C is shown immediately, along with the practical settling times. After one τ the capacitor has charged (or discharged) to about 63.2% of its final value; 3τ reaches roughly 95% and 5τ about 99.3%. These multiples are the rule of thumb for judging "when is it done" in any RC circuit, so the calculator reports them all at once.

What the value means in a circuit

The time constant sets how fast an RC network responds — it is the basis of charge/discharge timing, debounce circuits, leaky integrators, and the corner frequency of first-order filters (f = 1 / 2πRC). A larger R or C gives a slower response (longer τ), while a smaller product reacts faster. Because nearly every timing and filtering application hinges on τ, getting it right tells you both the delay and the bandwidth of the circuit.