Time constant τ calculation

Online calculator and formulas for calculating the time constant τ (tau) of resistor and capacitor


On this page you can calculate the time constant of a resistor and a capacitor.

The time constant of an RC element (low pass) is the product of R * C. Your unit of measure is the second. The formula symbol is the Greek letter τ (tau).

The time constant is required to calculate the state of charge at a specific point in time when the capacitor is being charged or discharged.

After a time of approx. 3 time constants, the output signal has reached approx. 95% of the size of the input signal. After 5 τ the charge is approx. 99.3%.

Online calculator

Enter the two values in the appropriate fields.

Note the unit of measurement



Time constant

Decimal places

Formula for calculating the time constant

\(\displaystyle τ=R ·C\)


\(\displaystyle R\)


\(\displaystyle C\)


\(\displaystyle τ\)

Time constants