Interactive LC Filter Calculator
Use this calculator for ideal LC filter/resonant calculations. Pick what you want to solve, enter known values, and click Calculate.
f₀ = 1 / (2π√(LC))
What Is an LC Filter?
An LC filter is a circuit made from an inductor (L) and a capacitor (C). Together, they create a frequency-selective network that can pass, reject, or shape parts of a signal. You will find LC filters in power supplies, radio front-ends, audio crossovers, sensor interfaces, and RF tuning circuits.
The key idea is simple: inductors and capacitors react differently to frequency. By combining them, you can target a specific cutoff or resonant point and build low-pass, high-pass, band-pass, or band-stop behavior.
Core Equations Used by This Calculator
1) Resonant (or natural) frequency
For an ideal LC network, the natural frequency is:
f₀ = 1 / (2π√(LC))
2) Solving for inductance
L = 1 / ((2πf₀)² · C)
3) Solving for capacitance
C = 1 / ((2πf₀)² · L)
4) Characteristic impedance
When both L and C are known, the calculator also reports:
Z₀ = √(L/C)
How to Use the Filter LC Calculator
- Select what you want to compute: frequency, inductor, or capacitor.
- Enter the known values and choose the proper units (kHz, µH, nF, etc.).
- Click Calculate.
- Read the full result set: f₀, ω₀, L, C, and Z₀.
Tip: unit mistakes are the most common source of bad designs. Always confirm whether your values are in milli-, micro-, nano-, or pico-scale before building hardware.
Quick Design Example
Suppose you want a resonant point around 100 kHz and you already selected a 10 nF capacitor. Solve for inductance:
- f₀ = 100 kHz
- C = 10 nF
- Calculated L ≈ 253 µH
In practice, choose the nearest standard inductor value, then re-check the actual resulting frequency. Small value changes can shift your center/cutoff frequency noticeably.
Practical LC Filter Design Notes
Real components are not ideal
Inductors have winding resistance (DCR), and capacitors have ESR and ESL. These parasitics reduce Q, introduce loss, and can move the real response away from ideal math.
Source and load matter
The simple equations assume an ideal or lightly loaded condition. In real circuits, source impedance and load impedance alter filter shape and effective cutoff. For precision work, simulate with SPICE and include real component models.
Tolerance and drift
- Capacitor tolerance (e.g., ±5%, ±10%) changes cutoff directly.
- Inductor tolerance and core effects can be significant.
- Temperature and DC bias may shift values in operation.
When to Use Low-Pass, High-Pass, or Band-Pass LC Networks
- Low-pass: remove high-frequency noise/ripple while keeping lower frequencies.
- High-pass: block DC or low-frequency drift and pass higher-frequency content.
- Band-pass: isolate a narrow band (common in RF and communications).
Final Thoughts
This filter LC calculator gives fast first-pass values for design and learning. It is perfect for quick sizing and sanity checks. For final production designs, pair these calculations with simulation, component datasheets, and bench measurements.