RC Low Pass Filter Calculator

What is an RC Low-Pass Filter?

An RC low-pass filter allows signals below a cutoff frequency to pass through while attenuating higher frequencies. It's one of the simplest and most widely used filter circuits, consisting of just a resistor and capacitor.

The Cutoff Frequency Formula

fc = 1 / (2π × R × C)

At the cutoff frequency (also called −3dB frequency), the output amplitude is 70.7% of the input (−3dB). Above this frequency, the attenuation increases at 20dB per decade.

How to Use This Calculator

Enter R and C to find the cutoff frequency, or enter the desired cutoff frequency and one component to find the other. The calculator also shows attenuation at any frequency.

Practical Examples

Example 1: R = 10kΩ, C = 10nF. fc = 1 / (2π × 10,000 × 0.00000001) = 1,592 Hz ≈ 1.6 kHz.

Example 2: Audio low-pass for subwoofer, cutoff at 200 Hz. Using C = 100nF: R = 1 / (2π × 200 × 0.0000001) = 7,958Ω → use 8.2kΩ.

Example 3: Anti-aliasing filter before a 44.1kHz audio ADC, cutoff at 20kHz. R = 1kΩ, C = 8.2nF gives fc = 19.4 kHz.

Applications

• Audio: Tone controls, speaker crossovers, noise filtering
• Power supplies: Smoothing rectified DC
• ADC anti-aliasing: Preventing high-frequency aliasing in digitisers
• Signal conditioning: Removing noise above the signal band