EMBEDDED TOOL
RC Time Constant Calculator
Calculate resistor-capacitor time constant, cutoff frequency, and charge-time milestones.
Time constant (τ = R × C)
—
Cutoff frequency (−3dB, f = 1/2πRC)
—
Hover the chart to read exact values. Shows charging from 0V toward a 1V supply through the resistor.
About this tool
Calculate the RC time constant (τ = R × C) for a resistor-capacitor pair, along with the −3dB cutoff frequency if it's being used as a filter, and a reference table of how long it takes to charge or discharge to common percentages — handy for debounce timing, filter design, and timing circuits.
Frequently asked questions
- What does "one time constant" actually mean?
- After 1τ, a charging capacitor reaches about 63.2% of its final voltage (and a discharging one drops to 36.8% of its starting voltage) — it's an exponential approach, so it never mathematically reaches 100%, which is why the table shows milestones like 5τ (99.3%) as "practically fully charged" instead.
- How is the cutoff frequency useful?
- For an RC low-pass or high-pass filter, f = 1/(2πRC) is the frequency at which the output has dropped by 3dB (about 70.7% of input amplitude) — the frequency that separates the "passed" and "attenuated" regions of the filter.
- What's a typical RC value for a button debounce circuit?
- Something in the 1–10ms range works well for most mechanical switches — e.g. a 10kΩ resistor with a 100nF–1µF capacitor gives a time constant in that range, long enough to smooth out contact bounce without noticeably delaying a real button press.