What determines the steepness?

Filter order determines roll-off steepness. First-order (RC, RL) gives -20 dB/decade. Second-order (LC) gives -40 dB/decade. Higher orders require cascading or active filter designs.

How do I choose component values?

Start with the desired fc. Choose a convenient R (typically 1k-100k for signal filters), then C = 1/(2πRfc). Or choose C first and calculate R. Standard component values may shift fc slightly.

Does a low pass filter affect DC?

No. A low pass filter passes DC (0 Hz) without attenuation. Only frequencies above fc are attenuated. This is why low pass filters are used for power supply smoothing.

Low Pass Filter Calculator

Passes low frequencies

Attenuates high frequencies

−3 dB at f_c

RC Low Pass Filter

Ω Resistance

C Capacitance

Enter component values

Low Pass Filter Results

Cutoff fc
—
Hz

ω_c

—

rad/s

Roll-off

—

Gain at 10×f_c

—

Gain at 100×f_c

—

Time Constant

—

Figure 1: A low pass filter passes frequencies below f_c and progressively attenuates frequencies above it. The transition steepness depends on filter order.

Table of Contents

Fundamentals

What Is a Low Pass Filter?
How It Works

Worked Examples

Anti-Aliasing Before ADC
Power Supply Smoothing

Deep Dive

Choosing Between RC, RL and LC
Cascading for Steeper Roll-off

Reference

Frequently Asked Questions
Related Filter Calculators

What Is a Low Pass Filter?

A low pass filter passes frequencies below its cutoff frequency and progressively attenuates frequencies above it. It is the most common filter type, used for noise removal, anti-aliasing, power supply smoothing, and audio tone shaping. The Filter Cutoff Frequency Calculator covers the fundamental cutoff frequency calculation that defines every low pass filter.

How It Works

Low Pass Filter: f_c = −3 dB point where output power drops to 50%.
Below f_c: passband | Above f_c: stopband

The filter response is determined by the component values and circuit topology. First-order filters (single RC or RL) give −20 dB/decade roll-off. Second-order (LC or cascaded) give −40 dB/decade. The RC Filter Calculator shows how different component combinations affect the response.

Worked Example — Anti-Aliasing Before ADC

Given: R = 10 kΩ, C = 100 nF

f_c = 1/(2π × 10000 × 100×10⁻⁹) = 159.2 Hz

Frequencies below 159 Hz pass; above are attenuated.

The RL Filter Calculator designs the complementary response.

Worked Example — Power Supply Smoothing

Given: R = 1 kΩ, C = 1 µF

f_c = 1/(2π × 1000 × 10⁻⁶) = 159.2 Hz

Choosing Between RC, RL and LC

For low-pass filtering, RC is simplest for signals, RL for power, and LC for sharp cutoff without loss. The LC Filter Calculator handles the complementary filter type.

Cascading for Steeper Roll-off

Cascading identical first-order stages doubles the roll-off rate but shifts the overall fc. Use Butterworth or Chebyshev designs for optimised multi-order responses. The High Pass Filter Calculator covers the active version with op-amp gain.

RLC Circuit Calculator: The RLC Circuit Calculator analyses the full RLC response including damping and transient behaviour.

Frequently Asked Questions

What is the −3 dB point?

−3 dB is where the output power is half the input power (voltage is 70.7% of input). This is the universally agreed definition of cutoff frequency for all filter types.

Can I use this for audio?

Yes. Audio frequencies range from 20 Hz to 20 kHz. Choose component values that place f_c within this range for audio filtering applications.

What order filter do I need?

First-order (−20 dB/dec) is adequate for gentle roll-off. For sharper transitions, use second-order (−40 dB/dec) or higher. The Butterworth Filter Calculator designs multi-order responses.

Does the filter introduce delay?

Yes. All filters introduce phase shift and group delay. First-order filters have constant group delay (linear phase approximation). Higher-order filters can have significant group delay variation near fc.

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Last updated: March 2026