RC Time Constant Calculator
Charge time and cutoff frequency.
Reviewed by James Whitfield · How we check our maths
Your values
ⓘ Live results
Results
- Time constant
- 1 s
- Time to 99% charge
- 5 s
- Voltage after 1τ
- 7.58 V
- Cutoff frequency
- 0.16 Hz
Quick answer
One RC time constant charges a capacitor to 63% of the supply voltage.
The formula
τ = R × C
Worked example
Worked example
Using the values the calculator loads with.
Inputs
- Resistance10 kΩ
- Capacitance100 µF
- Supply voltage12 V
Result
- Time constant1 s
- Time to 99% charge5 s
- Voltage after 1τ7.58 V
- Cutoff frequency0.16 Hz
How to use the RC Time Constant Calculator
- 1Enter resistance in kilohms.
- 2Enter capacitance in microfarads.
- 3Enter supply voltage.
Frequently asked questions
Why five time constants?
After 5τ the capacitor is over 99% charged, treated as fully charged.
Is discharge the same?
Yes — it falls to 37% after one time constant.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error