Interactive LC Filter Calculator
Calculate cutoff frequency, inductance, or capacitance for an ideal LC network. This tool uses the common resonance relationship:
- fc = 1 / (2π√(LC))
- L = 1 / ((2πf)2 C)
- C = 1 / ((2πf)2 L)
- Z0 ≈ √(L/C) (ideal characteristic impedance)
If provided, the calculator estimates loaded Q using Q ≈ R√(C/L).
What this LC filter calculator helps you do
An LC filter calculator speeds up one of the most common electronics design tasks: choosing inductor and capacitor values to hit a target cutoff or resonant frequency. Whether you are tuning a low-pass output stage for a switching regulator, creating an RF tank, or prototyping a high-pass front end, this tool gives you fast first-pass values in seconds.
The calculator is built for practical use with engineering units (µH, nF, kHz, and more), and it also reports ideal characteristic impedance. If you provide a load resistance, it adds a quick Q estimate so you can see whether your response may be underdamped or peaky.
LC filter basics in plain language
Inductors resist changes in current
An inductor stores energy in a magnetic field. At higher frequencies, its reactance rises, so it tends to block rapidly changing current.
Capacitors resist changes in voltage
A capacitor stores energy in an electric field. At higher frequencies, its reactance drops, so it tends to shunt high-frequency energy to ground in many low-pass topologies.
Put them together, and you get frequency selectivity
Because L and C react in opposite ways with frequency, their combination creates a selective network. Around resonance, energy swaps between magnetic and electric fields, producing a natural frequency that sets the heart of the filter behavior.
Key equations used by this calculator
- Resonant/cutoff estimate: f = 1 / (2π√(LC))
- Solve for L: L = 1 / ((2πf)2C)
- Solve for C: C = 1 / ((2πf)2L)
- Characteristic impedance (ideal): Z0 = √(L/C)
These are ideal equations. Real filters include source/load impedance, inductor DCR, capacitor ESR, parasitics, and layout effects. Use these results as an engineering starting point, then validate with simulation and measurement.
Typical design workflow
1) Pick your target cutoff frequency
Start from system requirements. For a power supply output filter, choose a cutoff far below the switching frequency. For signal chains, choose based on required passband and attenuation.
2) Choose a practical component first
Designers often pick either L or C based on part availability, size, cost, or current rating. Then solve for the other component using the calculator.
3) Check impedance and damping
Use Z0 and optional Q estimate to catch obvious issues early. Very high Q can cause ringing; very low Q can over-damp and reduce selectivity.
4) Validate with non-ideal models
Run SPICE with ESR/DCR and expected load. Then bench test with realistic wiring and measurement bandwidth.
Example: quickly sizing a low-pass LC stage
Suppose you want an ideal cutoff near 50 kHz and already selected a 22 µH inductor. Enter:
- Mode: Find capacitance (C)
- L = 22 µH
- fc = 50 kHz
The calculator returns a capacitance close to 460 nF (ideal). You might then choose a nearby standard value and re-check behavior with ESR and load variation.
Practical tips for better real-world results
- Inductor current rating matters: avoid saturation at peak current.
- Watch DCR: higher DCR adds loss and can change damping.
- Capacitor dielectric matters: X7R can lose capacitance under bias; film can be more stable.
- ESR can help or hurt: a little ESR may damp ringing, too much reduces filtering effectiveness.
- Layout is part of the circuit: short current loops and proper grounding improve high-frequency behavior.
Low-pass vs high-pass LC networks
The same resonance equation appears in both low-pass and high-pass LC topologies, but the placement of L and C is different, so the transfer function and load interaction differ. Use this calculator for core value estimation, then apply topology-specific analysis for final tuning.
Frequently asked questions
Is this calculator for RF circuits too?
Yes, for first-order value estimation. In RF design, parasitics and component self-resonance become critical, so electromagnetic layout and S-parameter verification are essential.
Why does measured cutoff differ from the calculated value?
Most often due to source/load impedance, ESR/DCR, tolerance, PCB parasitics, and measurement setup. Ideal formulas assume perfect parts and ideal terminations.
Can I use this as a low-pass filter calculator for switching power supplies?
Absolutely. It is a useful starting point for buck/boost output filtering. Just remember to include ripple current limits, transient response targets, and control-loop stability in your final design.
Bottom line
This LC filter calculator is designed to give you fast, practical component estimates with minimal friction. Use it to move quickly from concept to prototype, then refine with simulation and bench data for production-ready performance.