RC Time Constant Calculator
R × C sets the speed — compute τ, the settling time, and the cutoff frequency.
What it does: Compute the time constant and cutoff frequency of an RC circuit from the resistance and capacitance.
When to use it: When building delays, debouncing, RC filtering, or estimating charge/discharge speed.
MEANS About — to charge to 63%, about — to be considered fully charged; used as a low-pass filter, the corner frequency is about —.
Common cases pre-computed: browse RC time constant presets →
No history yet. Each calculation is automatically saved to this device.
How to use the RC time constant calculator
Just enter R and C.
- 01
Enter the resistance R
The resistance in the charge/discharge path; accepts
1k,10k,470. - 02
Enter the capacitance C
The energy-storing capacitor; accepts
1u,100n,10uF. - 03
Read τ and the cutoff frequency
τ = R×C is the time to charge to 63%; about 5τ is considered fully charged; fc is the corner frequency of the RC low-pass.
Charge progress reference (in multiples of τ)
Capacitor charging follows an exponential curve, getting closer to the target with each τ that passes.
| Elapsed time | Fraction of supply voltage reached |
|---|---|
| 1τ | 63.2% |
| 2τ | 86.5% |
| 3τ | 95% |
| 4τ | 98.2% |
| 5τ | 99.3% |
Fractions computed from 1 − e^(−t/τ).
Common questions, answered in 3 minutes
What exactly is the time constant τ?
τ = R×C, in seconds. It is the time for a capacitor to charge through a resistor to about 63.2% (or discharge to 36.8%), and it is the single measure of how fast an RC circuit responds.
Why is it often said that "5 τ" is fully charged?
At 5τ the capacitor has charged to about 99.3%, which engineering treats as fully charged. In theory it never reaches 100%, but the difference is negligible.
What is the cutoff frequency fc good for?
When using an RC as a low-pass filter, fc = 1/(2πRC) is the −3 dB corner point: signals below it pass largely intact, signals above it are attenuated.
Does the supply voltage affect charging?
It does not affect τ. τ depends only on R and C; the supply voltage only sets the final voltage amplitude reached, not the speed.
Standards and sources referenced by this tool
| Item | Value / Formula | Source |
|---|---|---|
| Time constant | τ = R × C | First-order RC circuit |
| Cutoff frequency | fc = 1 / (2π·R·C) | RC low-pass −3 dB point |
| Step response | v(t) = V·(1 − e^(−t/τ)) | Exponential charging |
Pure formula calculation, no external API.