RC Time Constant Calculator

Calculate the RC time constant

The RC time constant determines how quickly a capacitor charges or discharges through a resistor. It is simply the resistance multiplied by the capacitance, giving a result in seconds. In one time constant a charging capacitor reaches about 63.2 percent of the supply voltage, and after five time constants it is considered fully charged at over 99 percent. This calculator computes the time constant and the practical five-time-constant charging time. A ten-kilohm resistor with a one-hundred-microfarad capacitor gives a time constant of one second.

The same time constant governs discharge, where the capacitor loses 63.2 percent of its charge in one time constant.

Timing with RC circuits

Resistor-capacitor circuits are everywhere in electronics, used for timing, filtering, smoothing and debouncing. The time constant sets the pace of these behaviours: a larger resistance or capacitance slows the response, while smaller values speed it up. Designers choose resistor and capacitor values to achieve a desired delay or filter cut-off, making the time constant a key design parameter.

Because capacitance values are often very small, the time constant can span a huge range, so the calculator uses scientific notation where helpful. All calculation happens locally in your browser.

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