rlc filter calculator

What this RLC filter calculator does

This calculator helps you analyze a classic series RLC network quickly, without doing repetitive hand calculations. Whether you are tuning a narrow band-pass stage, checking a low-pass roll-off shape, or inspecting a high-pass response, you can enter your resistor, inductor, and capacitor values and immediately see the key electrical metrics.

The tool computes the resonant frequency, quality factor, damping ratio, estimated -3 dB points (for band-pass interpretation), and the response magnitude at a user-selected test frequency. It also reports reactances and phase angle so you can understand how the circuit behaves both near and far from resonance.

Quick refresher: RLC filter behavior

Resonance

In a series RLC circuit, resonance happens when inductive reactance equals capacitive reactance, so XL = XC. At that point, reactive effects cancel and the circuit impedance is mostly resistive. The resonant frequency is:

f₀ = 1 / (2π√(LC))

Quality factor and bandwidth

The Q factor indicates how selective the circuit is around resonance. Higher Q means a sharper, narrower response. For a series RLC circuit:

• Q = (1/R)√(L/C)
• BW = f₀ / Q

If you are using the resistor as output (band-pass mode), these values are especially useful for understanding passband shape.

How to use the calculator

• Select the output type: band-pass (across R), low-pass (across C), or high-pass (across L).
• Enter R in ohms, L in millihenries, and C in microfarads.
• Optionally enter a frequency to evaluate gain/phase at a specific point.
• Click Calculate to view all computed results.

If you leave analysis frequency blank, the calculator automatically evaluates response at resonance.

Interpreting your results

Resonant frequency (f₀)

This is the center point where energy exchange between L and C is maximal. In band-pass mode, the response generally peaks near this value.

Q and damping ratio

Damping ratio is shown as ζ = 1/(2Q). A low damping ratio corresponds to a peaky response; a higher damping ratio means flatter and broader behavior.

Gain magnitude and dB

Gain is given both as a linear ratio and in dB. This is useful when comparing practical attenuation or amplification stages in larger analog signal chains.

Practical design tips

• Component tolerance matters: 5% capacitors and inductors can shift center frequency significantly.
• Inductor resistance and ESR: Real inductors add loss, lowering effective Q.
• Loading effects: The next stage can change filter shape; buffer if needed.
• Frequency range: At very high frequencies, parasitic capacitance/inductance can dominate.
• Simulate first: Use SPICE to verify real-world behavior before PCB production.

Worked example

With the default values R = 100 Ω, L = 10 mH, and C = 0.1 µF, the resonant frequency lands in the low kilohertz range. If you switch to band-pass mode and evaluate at resonance, you should see strong pass behavior and near-zero phase shift in the total impedance angle. Increasing R will reduce Q and widen bandwidth; decreasing R does the opposite.

Final note

This calculator is ideal for quick design iteration and learning. For precision hardware work, always validate with measured parasitics and bench testing.