RC Time Constant Calculator

This RC Time Constant Calculator works out the time constant of a resistor and capacitor circuit, which sets how quickly a capacitor charges or discharges. You enter the resistance in ohms and the capacitance in farads, and the tool returns the time constant, usually written as the Greek letter tau, along with the practical time to reach about full charge, which is five time constants. The time constant is simply resistance multiplied by capacitance, and it equals the time for the capacitor to reach about 63 percent of the supply voltage when charging, or to fall to about 37 percent when discharging. After five time constants the capacitor is more than 99 percent charged, so engineers treat that point as effectively full. This calculation underpins timing circuits, filters, debounce delays, smoothing in power supplies and the timing of simple oscillators, so it is one of the first things electronics students learn. People use it to choose component values for a wanted delay, to predict how fast a signal settles, and to design low pass and high pass filters. A few tips help you get reliable results. First, convert capacitance to farads, since parts are usually labelled in microfarads or nanofarads. Second, use the total resistance in the charge or discharge path, not just one resistor, when several are involved. Third, remember that a larger resistance or capacitance means a slower circuit, which is exactly what you want for longer delays. Used this way the calculator gives a fast, dependable estimate for timing and filter design.

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Tau = R x C. Time to about full charge = 5 x tau, in seconds.