Enter Values
Formula
τ = R × C
A capacitor reaches about 63.2% of its final voltage after one time constant and over 99% after five time constants.
Example
For 10 kΩ and 100 µF, τ = 1.00 second, and five time constants equal 5.00 seconds.
Reading an RC Time Constant
- At 1τ, a charging capacitor reaches about 63.2% of the final voltage.
- At 3τ, it reaches about 95%.
- At 5τ, it reaches more than 99% and is often treated as fully charged.
During discharge, the remaining voltage is about 36.8% after one time constant.
Real Component Effects
Actual timing can vary because of capacitor tolerance, leakage, equivalent series resistance, resistor tolerance, source resistance, and the input impedance of the connected circuit. Electrolytic capacitors often have wide capacitance tolerance.
Use component datasheets and worst-case tolerances when timing accuracy matters.