LC Low Pass Filter Calculator
Calculate cutoff frequency (fc), required inductance (L), or required capacitance (C) using the ideal LC relationship.
What this LC LPF calculator does
This tool helps you design a simple LC low-pass filter using the standard ideal formula: \( f_c = \dfrac{1}{2\pi\sqrt{LC}} \). In practical terms, it tells you where your filter starts significantly attenuating higher-frequency signals.
You can use the calculator in three directions:
- Find cutoff frequency from known L and C.
- Find required L from target fc and known C.
- Find required C from target fc and known L.
Quick refresher: LC low-pass filter basics
A common passive LC low-pass arrangement uses an inductor in series with the signal path and a capacitor to ground. At low frequencies, the inductor has low reactance and the capacitor has high reactance, so the signal passes. At high frequencies, the inductor impedance rises and capacitor impedance drops, shunting high-frequency content away from the load.
Core equations
- Cutoff frequency: fc = 1 / (2π√(LC))
- Inductance: L = 1 / ((2πfc)²C)
- Capacitance: C = 1 / ((2πfc)²L)
How to use this calculator
1) Pick calculation mode
Select whether you want to compute cutoff frequency, inductance, or capacitance. The calculator automatically disables the field being solved for.
2) Enter known values and units
You can mix engineering units (mH, uH, nF, kHz, MHz, etc.). The calculator converts everything to SI internally.
3) Click calculate
The result appears in an engineering-friendly format with a secondary SI value for reference.
Example design scenarios
Example A: Find cutoff frequency
Suppose you choose L = 10 mH and C = 100 nF. The calculator returns a cutoff near 5.03 kHz, useful for removing high-frequency noise above the audio band.
Example B: Find inductor value
If you want fc = 2 kHz and already have C = 220 nF, the required inductor is around 28.8 mH (ideal).
Example C: Find capacitor value
For fc = 100 kHz with L = 47 uH, the capacitor target is approximately 53.9 nF.
Practical tips for real-world LC filter design
- Choose components with tolerance in mind (e.g., ±5%, ±10%).
- Check inductor current rating and saturation current.
- Look at capacitor voltage rating and dielectric type (C0G, X7R, film, etc.).
- For power applications, watch ESR and ripple current heating.
- Validate performance with SPICE simulation and then bench measurements.
Common mistakes
- Mixing units (e.g., entering uH as mH by accident).
- Ignoring load impedance effects.
- Assuming ideal response without considering parasitics and layout.
- Expecting exact cutoff from wide-tolerance parts.
Final note
This LC LPF calculator is a fast starting point for electronics design, prototyping, and study. Use it to narrow component choices quickly, then validate with your actual circuit conditions.