RC time constant & filter calculator
Enter a resistor and capacitor to get the RC time constant (τ), how long the capacitor takes to charge, and the cut-off frequency of a simple RC low-pass or high-pass filter. Add a signal frequency to see how much gets through.
Enter your values
Find how fast a capacitor charges through a resistor (τ), and the cut-off frequency of a simple RC low-pass or high-pass filter.
Result
τ = R × Cfc = 1 ÷ (2π × R × C)V(t) = V × (1 − e^(−t/τ))Coro's tip: After 1τ a capacitor reaches 63% of the supply. After 5τ it is about 99% — call it fully charged. At fc the filter lets through about 71% (−3 dB).
RC time constant & filter calculator: common questions
What is the RC time constant?
τ = R × C, in seconds when R is in ohms and C in farads. After one τ a charging capacitor reaches about 63% of the supply voltage.
How long does a capacitor take to fully charge?
About 5 time constants (5τ), when it reaches roughly 99% of the supply.
What is the cut-off frequency of an RC filter?
fc = 1 ÷ (2π × R × C). At fc the output is about 70.7% of the input (−3 dB).
For learning and low-voltage hobby circuits (12 V and below). Always check the part's datasheet. Never use these tools to plan mains (230 V) wiring.