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Passive filter calculator

Low-pass & high-pass filter calculator

Calculate RC, RL and ideal LC cutoff or reference frequency, evaluate low-pass and high-pass response, or solve backward for a target component value.

Isometric illustration of a passive RC filter between two waveforms

Filter inputs

Evaluate RC, RL or ideal LC networks, or solve for a target cutoff.

Calculated response

Cutoff frequency

1.5915 kHz

Resistance

10 kΩ

Capacitance

10 nF

Time constant

100 µs

Gain at 10 kHz

-16.07 dB

Good first pass

First-order passive reference. The response is -3.01 dB at cutoff and changes by 20 dB per decade.

0 dB−20−400.01fcfc100fc

How to use the filter calculator

Choose a network and response, then evaluate component values or solve for one component at a target frequency.

  1. 01

    Choose RC, RL or LC

    Select the ideal network model and low-pass or high-pass response that matches the circuit you are checking.

  2. 02

    Evaluate or design

    Evaluate calculates frequency and response from component values. Design solves for a selected R, C or L at a target frequency.

  3. 03

    Review response and assumptions

    Use the plot and spot values as first-order references, then account for source, load, tolerance, parasitics and damping.

Filter frequency equations

RC and RL models use first-order response equations; the LC result is an ideal undamped reference frequency.

RC cutoff

fc = 1 / (2πRC)

RL cutoff

fc = R / (2πL)

Ideal LC frequency

f0 = 1 / (2π√(LC))

First-order low-pass

|H| = 1 / √(1 + (f/fc)²)

First-order high-pass

|H| = (f/fc) / √(1 + (f/fc)²)

At cutoff, an ideal first-order RC or RL response is about −3.01 dB. The LC equation identifies the undamped natural frequency only; real LC response depends strongly on source resistance, load resistance, component loss and topology.

Treat results as ideal design references. Verify impedance, damping, component tolerance, ESR, winding resistance, self-resonance, voltage/current stress and source/load interaction before selecting production parts.

Practical filter checks

Confirm that the ideal network model matches the circuit around it.

Include source and load impedance

A source or load comparable to the selected component impedance shifts cutoff and changes passband gain.

Evaluate tolerance corners

Calculate minimum and maximum frequency from component tolerances when response limits matter.

Check non-ideal components

Capacitor ESR and leakage, inductor winding resistance and self-resonance can dominate outside a limited frequency range.

Treat LC as a reference model

An undamped LC frequency is not a complete filter design. Choose topology and damping from the required impedance and response.

Frequently asked questions

What does the −3 dB cutoff mean?

For an ideal first-order RC or RL filter, output magnitude at cutoff is 1/√2 of the passband amplitude, or about −3.01 dB.

Can the calculator include source and load resistance?

Not directly. Combine resistances only when circuit analysis shows they belong in the model; otherwise simulate or calculate the loaded network separately.

Is the LC result a complete low-pass or high-pass design?

No. It is the ideal undamped reference frequency. Real LC filters require topology, impedance and damping choices.

Why does the measured cutoff differ?

Component tolerance, parasitics, source/load impedance and instrument loading can all shift the measured response.

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Technical references: Texas Instruments RC circuits training, TI Basic Introduction to Filters