RC Filter Calculator

How to use

1. 1Choose the filter type: RC low-pass, RC high-pass, or LC resonant. The description and the available inputs update to match.
2. 2Pick what to solve for: the cutoff frequency, the resistor or inductor, or the capacitor. The field being solved for is greyed out.
3. 3Enter the other two values, choosing the right unit from each dropdown (for example kΩ for the resistor and nF for the capacitor).
4. 4Read the highlighted result plus the cutoff frequency, angular cutoff in rad/s, and, for RC filters, the time constant.
5. 5Click a worked-example preset to load a realistic audio, DC-blocking, or RF circuit, then edit the values to match your own design.
6. 6Use Copy summary to grab the full result as text, or Reset to return to the defaults.

About this tool

RC Filter Calculator works out the cutoff frequency of the three passive filters electronics designers reach for most, and it solves in any direction so you can also find the part value you need. Pick RC low-pass (a resistor feeding a capacitor to ground), RC high-pass (a capacitor feeding a resistor to ground), or LC (an inductor and capacitor forming a resonant corner or tank), then enter any two of the three relevant quantities and the third is calculated for you. The math is the standard textbook set: for an RC filter the cutoff is f_c = 1 / (2 pi R C), and because that single relationship ties the three values together the tool rearranges it to give R = 1 / (2 pi f_c C) when you know the frequency and the capacitor, or C = 1 / (2 pi f_c R) when you know the frequency and the resistor. The low-pass and the high-pass share the same corner frequency, so they share one formula here; the difference is only which side of the corner passes, which the reference table spells out. For an LC filter the resonant corner is f_c = 1 / (2 pi sqrt(L C)), rearranged to L = 1 / ((2 pi f_c) squared times C) or C = 1 / ((2 pi f_c) squared times L) when you are solving for a component. Every input has a unit dropdown so you type values the natural way: ohms, kilohms, megohms, or milliohms for resistance; farads down to picofarads for capacitance; henries down to nanohenries for inductance; and hertz up to gigahertz for frequency. Results are auto-scaled to a readable prefix, so a 10 kilohm resistor with a 1 nanofarad capacitor reports a cutoff of about 15.9 kHz rather than a long string of digits. Alongside the cutoff the tool always shows the angular cutoff in radians per second (omega_c = 2 pi f_c) and, for the two RC modes, the time constant tau = R C in seconds, with a note that the capacitor charges to roughly 63 percent of its final voltage in one time constant. Three worked-example presets load a realistic audio low-pass, a DC-blocking high-pass, and an RF LC tank so you can see sensible numbers immediately and then edit them. A short explainer covers the things people get wrong: a first-order RC filter rolls off at 6 dB per octave (20 dB per decade), the cutoff is the half-power point where the output is 3 dB down at about 70.7 percent of the input amplitude, and the phase shift there is 45 degrees. One honest limit: the LC result is the ideal undamped resonant frequency, real circuits carry series resistance that broadens the response, and standard E-series resistor and capacitor values mean the nearest stock part can shift the actual corner slightly from the exact figure. This calculator covers a single-pole (first-order) filter, not multi-pole Sallen-Key or Butterworth stages. Everything runs locally in your browser, so the component values you enter are never uploaded, logged, or stored.

Free to use. Works in your browser. No signup, no login.