RC Filter Calculator (Low-pass & High-pass)

Calculate the cutoff frequency (-3dB point) for RC circuits. Features E12/E24 resistor matching and simple Bode plot visualization for audio and electronics engineering.

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About

In analog electronics and signal processing, the RC filter is a fundamental building block used to permit certain frequencies to pass while attenuating others. Whether you are designing a subwoofer crossover (Low-pass) or removing DC offset from an audio signal (High-pass), determining the precise cutoff frequency is critical for signal integrity.

This tool calculates the cutoff frequency (fc), defined as the point where the signal power drops by half (-3dB). Uniquely, it also references standard E-series component values, helping hobbyists and engineers swap theoretical values for real-world resistors and capacitors found in parts bins.

rc filter cutoff frequency low pass filter high pass filter bode plot electronics tool

Formulas

The cutoff frequency is determined by the time constant τ (R × C). The formula is:

fc = 12πRC

Where R is Resistance in Ohms (Ω) and C is Capacitance in Farads (F).

Reference Data

Frequency BandTypical ApplicationFilter Type NeededCommon Component Range
Sub-Bass (20-60 Hz)Subwoofer CrossoverLow-passLarge C (10μF+), Medium R
Bass (60-250 Hz)Rhythm SectionBand-passCombination
Midrange (250Hz-4kHz)Vocals / TelephonyBand-passMedium C (100nF), Medium R
Presence (4k-6kHz)Clarity / DefinitionHigh-passSmall C, High R
Brilliance (6k-20kHz)Hiss RemovalLow-passSmall C (<10nF), Low R
Ultrasonic (>20kHz)RF Noise FilteringLow-passCeramic C (pF range)

Frequently Asked Questions

The -3dB point (cutoff frequency) is the frequency at which the output voltage amplitude is reduced to 70.7% of the input voltage. In terms of power, this represents a 50% reduction. It marks the boundary between the 'passband' and the 'stopband'.
This calculator is designed for passive first-order RC filters. While the fc formula often applies to the pole location of active filters (like Op-Amp circuits), active filters have gain and impedance characteristics not modeled here.
Theoretical calculations often result in values like 4,123 Ohms, which don't exist as standard components. The E-series suggestions provide the closest commercially available resistor values (e.g., 3.9k or 4.7k) to help you build the circuit physically.
Physically, yes. For a Low-pass filter, the Resistor is in series with the signal and the Capacitor goes to ground. For a High-pass filter, the Capacitor is in series and the Resistor goes to ground.