RC Time Constant Calculator
Enter any two of resistance, capacitance, and time constant — leave the third field blank to calculate it.
Inputs
Result
Step-by-step working
Formula
- τ = RC
- C = τ/R
- R = τ/C
τ is the time constant in seconds, R is resistance in ohms, C is capacitance in farads.
About this calculation
In a circuit with one resistor and one capacitor, the time constant τ = R × C is the time for the capacitor to charge to about 63% of the applied voltage, or to fall to about 37% when it discharges. Engineers treat a capacitor as effectively fully charged after about 5τ, and the time constant sets the corner frequency of an RC filter. Enter any two of R, C and τ and the calculator finds the third.
Assumptions and limits
- It models a single resistor and a single capacitor with ideal values.
- Charging is assumed to start from an ideal step, with the supply able to deliver whatever current the capacitor initially asks for.
- Real capacitors leak, have equivalent series resistance, and show dielectric absorption, none of which are modelled here.
A worked example
10 kΩ with 100 nF gives τ = 10 000 × 100 × 10⁻⁹ = 1 ms, so the capacitor passes 63% after 1 ms and is considered charged after about 5 ms.
See also Frequency / Period and Ohm's Law.