〽️RC Filter Calculator

About this tool

Enter a resistor and capacitor using shorthand like 1k and 100n, and choose low-pass or high-pass. The calculator shows the −3 dB cutoff frequency, the time constant τ and the 10–90% rise time, a table of gain at frequencies from a tenth to ten times the cutoff, and charging percentages. Enter a target cutoff to get the capacitor value needed with your resistor.

How RC filters and time constants work

A resistor and a capacitor together make the simplest filter. A capacitor's impedance falls as frequency rises, so in a low-pass filter, with the resistor in series and the capacitor to ground, high frequencies are shunted away while low frequencies pass. Swap them for a high-pass filter. The cutoff frequency, where the output has fallen to 70.7% of the input (−3 dB), is fc = 1 ÷ (2πRC). Beyond it, a first-order filter rolls off at 20 dB per decade, so ten times the cutoff is attenuated to about 10%. With 1 kΩ and 100 nF, fc is about 1.59 kHz. The same pair sets a time constant τ = RC, here 100 µs: the time a capacitor takes to charge to 63.2% of the supply through the resistor. After 5τ it is over 99% charged, and the 10–90% rise time is about 2.2τ. RC networks smooth PWM signals, debounce switches, remove noise from sensors, and set timing in circuits such as the 555. Honest limits: these are ideal first-order formulas; the source and load impedances shift the real cutoff, capacitor tolerance can be 10–20%, and steeper filters need more stages or op-amps. The 555 timer calculator, the decibel calculator and the capacitor code calculator are related.

Frequently asked questions