Cutoff Frequency Calculator

The cutoff (corner) frequency is where an RC filter's output drops to −3 dB.

How the Math Works

The cutoff frequency of an RC filter is calculated using the formula fc = 1 ÷ (2π·R·C), where R is resistance in ohms and C is capacitance in farads. This formula derives from the behavior of first-order RC circuits, where the cutoff frequency represents the point at which the output signal is attenuated by 70.7% (or -3dB) of the input signal. The constant 2π arises from the relationship between angular frequency and regular frequency in sinusoidal analysis of reactive components.

Practical Applications

To use this calculator practically, simply enter your resistor value in ohms and capacitor value in farads (or use the appropriate unit prefixes like kiloohms and microfarads). The calculator will automatically compute the cutoff frequency in hertz. This is essential when designing low-pass filters to remove high-frequency noise, high-pass filters to block DC components, or creating frequency-selective circuits for audio applications, signal processing, and power supply filtering.

Day-to-Day Use

Understanding cutoff frequencies helps explain why your smartphone's audio system can filter out background noise during calls, why your car's ignition system uses capacitors to suppress electrical interference, and how your home's power outlets need proper filtering to prevent electromagnetic interference from affecting your Wi-Fi or other electronics. This knowledge empowers you to make informed decisions when troubleshooting audio issues, selecting appropriate capacitors for LED lighting projects, or understanding the frequency limitations of wireless devices in your daily environment.

Worked example

1 kΩ, 1 µF → about 159 Hz.

FAQ