RC Filter Calculator

Time constant, cutoff frequency and settling time for a resistor and capacitor. The arithmetic behind every debounce, smoothing and simple filter circuit.

Resistance (ohms)

Capacitance (nF)

Time constant --

Shop resistors and capacitors

The formula

tau = R x C
cutoff frequency = 1 / (2 x pi x R x C)
settles in about 5 x tau

How it works

One time constant charges a capacitor to 63% of the way there. Five gets it to over 99%, which is what people mean by settled.

The same pair is a filter. Below the cutoff frequency signals pass; above it they are attenuated, gently at first — a single RC rolls off at 6 dB per octave, so it is a slope rather than a wall.

Typical uses: 10k and 100nF gives 1 ms and a 159 Hz cutoff, which is a reasonable switch debounce. For smoothing a PWM output into an analogue voltage you want the cutoff well below the PWM frequency, not near it.

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