LC Low Pass Filter Calculator
Enter any two values and leave the third blank. The calculator uses the ideal LC cutoff relation for a 2nd-order low-pass section:
What this LC low pass filter calculator does
This tool quickly solves the core equation behind an ideal LC low-pass filter. In practical terms, it helps you pick inductor and capacitor values for a target cutoff frequency, or find the cutoff from component values you already have. It is useful for audio, RF front-end filtering, power electronics ripple reduction, and quick hand-checking of SPICE results.
The core formula
For an ideal second-order LC section, the natural cutoff (or resonant corner) is:
Rearranging gives:
- L = 1 / ((2πfc)² C)
- C = 1 / ((2πfc)² L)
The calculator applies these relationships directly with proper unit conversion (Hz/kHz/MHz, uH/mH, nF/uF/pF, and more).
How to use it
1) Enter any two values
Fill two fields only: frequency + inductance, frequency + capacitance, or inductance + capacitance. Leave the unknown field empty.
2) Select units carefully
Unit mistakes are the #1 source of wrong filter designs. Double-check uH vs mH and nF vs uF before calculating.
3) Click calculate
The missing value is computed and automatically inserted in the empty field using the unit currently selected for that field. The result panel also shows frequency, inductance, and capacitance in engineering notation.
Design notes for real-world filters
Ideal equations are a starting point. Actual LC filter performance depends on source impedance, load impedance, inductor series resistance (DCR), capacitor ESR, and component tolerance.
- Inductor DCR reduces Q and adds insertion loss.
- Capacitor ESR/ESL shifts high-frequency behavior and damping.
- Tolerances move the effective cutoff. Use tight components for precision work.
- Load interaction can significantly alter the response shape and corner frequency.
Example
Suppose you want approximately 5 kHz and have an inductor of 1 mH. Enter 5 kHz and 1 mH, leave C blank, and calculate. The ideal capacitor is about 1.01 uF. In practice you might choose a standard 1.0 uF value and then verify the final response in simulation and measurement.
When to use this calculator
- Quick component sizing for passive low-pass prototypes
- Checking if existing L/C values support your desired corner
- Education and concept validation before full filter synthesis
FAQ
Is this a Butterworth/Chebyshev synthesis tool?
No. This is a direct ideal LC cutoff calculator. For strict passband ripple and stopband attenuation targets, use full filter prototype tables and impedance scaling.
Can I use it for RF?
Yes, for initial estimates. At RF, parasitics and PCB layout become dominant quickly, so always validate with EM-aware simulation and measurement.
Why does measured cutoff differ from calculated cutoff?
Because real components and source/load impedances are not ideal. The equation is still valuable for first-pass sizing, but practical tuning is normal and expected.