First-Order Filter Calculator (RC / RL)
The simplest passive filter: one resistor plus one reactive element. A series element and a shunt element form a single-pole low-pass or high-pass with a −3 dB corner at the cutoff frequency and a gentle 20 dB/decade roll-off. Pick RC or RL, enter the two component values, and read the cutoff.
Equations & Parameters ▸
\(f_c^{RC} = \dfrac{1}{2\pi RC}\qquad f_c^{RL} = \dfrac{R}{2\pi L}\qquad \tau_{RC}=RC,\ \ \tau_{RL}=\dfrac{L}{R}\)
| Response | Low-pass passes DC and blocks high frequencies; high-pass does the reverse. |
| RC / RL | Choose a resistor–capacitor or resistor–inductor pair. |
| f_c | Cutoff (−3 dB corner) frequency. RC: 1/(2πRC). RL: R/(2πL). |
| τ | Time constant. RC: RC. RL: L/R. Related to f_c by f_c = 1/(2πτ). |
| Roll-off | A single pole gives 20 dB/decade (6 dB/octave) in the stopband. |
Note: f_c assumes an ideal source and open-circuit (unloaded) output. Source and load resistance shift the corner.
Inputs
Results
Cutoff
Cutoff frequency, f_c—
Angular cutoff, ω_c—
Time constant, τ—
Response
Topology—
Stopband slope—
Gain at corner—
Diagram