- Published 29 Aug 2025
- Last Modified 29 Aug 2025
- 2 min
Time Constant Calculator
Need to know how to calculate time constant? Our handy interactive tool can help provide an accurate figure, whether you’re trying to determine what is the time constant of a capacitor or need to use an RC circuit time constant calculator for your project.
To use our free time constant RC calculator, input the Voltage (V), Capacitance (C), and Load Resistance (R) into the interactive boxes below. You can also choose from a variety of common units of measurement for each input, allowing even greater flexibility.
Then, all that’s left to do is click the ‘calculate’ button, and our RC time constant circuit tool will instantly provide you with the outputs you need – Time Constant (τ) and Energy (E).
Understanding RC Time Constant
RC time constant is equal to the product of the capacitance and resistance values within a resistor-capacitor (RC) circuit. The time constant for a capacitor appears in the equation that describes a capacitor charging or discharging through a resistor.
The figure shows how long it takes for the voltage across the capacitor to reach roughly 63.2% of its fully-charged value after a voltage change is applied to an applicable circuit. In other words, the capacitor will have charged to 63.2% of its final value after one time constant.
It also calculates the total amount of energy stored in a capacitor when charged to a given voltage.
As a general rule, a capacitor is typically considered to be charged after a time period equal to three time constants (95% charged). The exception is mainly for high-precision circuits where the charging period needs to be increased to allow for a greater level of accuracy. For instance, after a period of time equal to five time constants, a capacitor should then be charged to over 99%.
Calculating Time Constant
It is possible to determine the time constant of a capacitor using the capacitance and load resistance values, with the equation below:
Time Constant (τ) = Resistance (R) x Capacitance (C)
You can also determine the capacitor’s stored energy by providing voltage and capacitance as your inputs, as per the following formula:
Energy Stored (E) = Voltage² (V²) x Capacitance (C) / 2
