Electronics CalculatorsRC time constant calculator
Enter the resistor and the capacitor. You get the time constant and the cut-off frequency of the RC filter.
The formulas
- Time constant: τ = R × C, in seconds when R is in ohms and C in farads.
- Cut-off frequency of a simple RC filter: f = 1 / (2π R C).
- Rise time from 10% to 90%: ln(9) × τ, which is about 2.197 τ.
- Time to reach 99%: ln(100) × τ, about 4.6 τ.
After one time constant a charging capacitor is at about 63% of the final voltage.
Writing capacitor values
Use u for micro, n for nano and p for pico: 100n is 100 nanofarad (0.1 µF), and 2u2 is 2.2 µF.
How it was tested
300 random R and C pairs, from 10 ohm to 1 megaohm and 10 picofarad to 1 millifarad, were compared with direct calculation.
Is this the cut-off of a high-pass or low-pass?
The same value applies to a simple RC low-pass and high-pass filter: it is the frequency where the output is about 70.7% of the input.
Does the tool include the tolerance of parts?
No. Real resistors and especially capacitors can be off by 5 to 20 percent or more: check their data sheets.
This is an independent guide. It is not connected with any standards body or company named here. The formulas are the usual textbook ones; the colour code, E-series and AWG values are those printed in common references, because the standards IEC 60062, IEC 60063 and ASTM B258 are paid documents that we did not read. Tables and examples were calculated with Python on 14 November 2026. Check data sheets and local safety rules before you build.