RC Filter Calculator

The RC filter is a direct consequence of Oliver Heaviside's 1890s impedance formalism: treat the capacitor as 1/(jωC) and analyze the circuit as a frequency-dependent voltage divider. At low frequency the capacitor's impedance is huge (acts open); at high frequency it's tiny (acts as a short). Put that in series or shunt with a resistor and you have a first-order filter whose −3 dB corner is fc = 1/(2πRC).

The filter became indispensable in early 20th-century long-distance telephony. George Campbell at AT&T in 1899 first used composite filter networks to split voice-band and carrier signals on shared copper lines; the resulting carrier telephone systems multiplied the capacity of a single pair by 24× by the 1920s. Filter synthesis matured rapidly in the 1930s through the work of Wilhelm Cauer and Otto Brune, and by WWII every radar, direction-finder, and radio receiver had dozens of RC, LC, and later active filter stages cascaded in its signal path.

The single-pole RC is still the most-used filter in the world. It's the integrator every microcontroller sits behind to anti-alias its ADC, the AC coupling cap on every audio amp input, the debouncer on every mechanical switch, and the ripple-smoothing cap downstream of every linear regulator. Its 6 dB/octave roll-off is gentle, but it's cheap, passive, unconditionally stable, and never drifts.

Pick low-pass (R in series, C to ground) or high-pass (C in series, R to ground), enter R and C with engineering suffixes, and this calculator returns the cutoff frequency, time constant, and log-log frequency response showing gain and phase. Or enter a target fc and the tool solves for C given the R you chose — useful for working with a standard resistor value.

The math assumes infinite load impedance at the output. For a real load comparable to R, the output sags; buffer with an op-amp follower or lower R so the load no longer matters. Everything runs in your browser; no values leave the page.