RC Time Constant and Filter Calculator
Time constant, settling time and cut-off frequency of a resistor-capacitor pair.
Results
Time constant
1.000000
s
Time
5.00000
s
Cut-off frequency
0.15915
Hz
Percentage
63.2
%
What this tool does
One time constant charges a capacitor to 63 % of the supply, and five of them get it to over 99 %, which is the usual definition of settled. The same R and C also set the corner frequency of a simple filter, which is why one calculation answers both questions.
Formula
τ = R × C · fc = 1 ÷ (2π × R × C)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
r | Resistance | kΩ |
c | Capacitance | µF |
T | Time constant | s |
T5 | Time | s |
FC | Cut-off frequency | Hz |
T63 | Percentage | % |
Worked example
- Resistance10 kΩ
- Capacitance100 µF
- Time constant1.000000 s
- Time5.00000 s
- Cut-off frequency0.15915 Hz
- Percentage63.2 %
Limitations
- The calculation runs at full precision and only the display is rounded. If you copy an intermediate value and retype it, small differences can appear.
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.