RC Filter Calculator | Low Pass & High Pass | Cutoff Frequency

📖 RC Filter Calculator

An RC filter is a simple passive filter using a resistor (R) and capacitor (C). This calculator helps you design low-pass and high-pass filters by calculating cutoff frequency, time constant, and frequency response.

📐 RC Filter Formulas

Cutoff Frequency: fc = 1 / (2π × R × C)
Time Constant: τ = R × C
Gain (Low Pass): H = 1 / √(1 + (f/fc)²)
Gain (High Pass): H = 1 / √(1 + (fc/f)²)
Phase Shift: φ = -arctan(f/fc) [LP] or φ = arctan(fc/f) [HP]

📊 RC Filter Applications

Application	Type	Typical fc
Audio Crossover	Low/High Pass	100 Hz - 5 kHz
Power Supply Filtering	Low Pass	10 Hz - 120 Hz
Signal Conditioning	Low Pass	1 Hz - 1 kHz
Noise Reduction	Low Pass	100 Hz - 10 kHz
DC Blocking	High Pass	0.1 Hz - 100 Hz
Audio Filters	Both	20 Hz - 20 kHz

❓ Frequently Asked Questions

What is cutoff frequency? ▼

Cutoff frequency (fc) is the frequency where the output signal is reduced to 70.7% (or -3dB) of the input. Below this frequency (LPF) or above this frequency (HPF), signals are attenuated.

What is the time constant of an RC circuit? ▼

The time constant τ = R × C is the time required for the capacitor to charge to 63.2% of the supply voltage. It determines how quickly the circuit responds to changes.

What is the difference between low-pass and high-pass filters? ▼

Low-pass filters pass frequencies below the cutoff frequency and attenuate higher frequencies. High-pass filters pass frequencies above the cutoff and attenuate lower frequencies.

How do I choose R and C values? ▼

Choose R and C based on fc = 1/(2πRC). For audio applications, use R in kΩ (1-100kΩ) and C in µF or nF. Larger R/C gives lower fc.