Low Pass Filter
Calculate RC and RL low-pass filter cutoff frequency with interactive Bode plots. Free online calculator for electronics and audio with gain analysis in dB.
About This Calculator
The Low Pass Filter Calculator computes the cutoff frequency, time constant, and gain for RC and RL passive low-pass filter circuits. This calculator is essential for electronics engineers, audio engineers, hobbyists, and students designing filter circuits for signal processing, audio crossovers, power supply smoothing, and noise reduction applications.
An RC low-pass filter consists of a resistor (R) and capacitor (C) in series, with the output taken across the capacitor. Its cutoff frequency is calculated using fc = 1 / (2π × R × C). An RL low-pass filter uses a resistor and inductor, with a cutoff frequency of fc = R / (2π × L). Both are first-order filters with a roll-off of -20 dB per decade above the cutoff frequency. At the cutoff frequency, the output signal is attenuated by 3 dB (to about 70.7% of the input voltage). The filter also imparts a phase shift that varies from 0° at DC to -90° at high frequencies, with -45° at the cutoff frequency.
Applications
Low-pass filters are fundamental in electronics and signal processing. Common applications include audio crossover networks where low frequencies are directed to woofers while blocking high frequencies from reaching them, anti-aliasing filters before analog-to-digital conversion to remove high-frequency noise, power supply smoothing filters that reduce ripple voltage after rectification, and RF circuits for rejecting high-frequency interference. In telecommunications, low-pass filters limit signal bandwidth and remove harmonics from transmitted signals.
Regional Notes
Low-pass filter design uses the same physics and formulas worldwide. Resistance is measured in ohms (Ω), capacitance in farads (F), and inductance in henries (H) — all SI base units used in India, the United States, the United Kingdom, and globally. The calculator supports common engineering unit prefixes (kΩ, MΩ, μF, nF, pF, mH, μH) through unit selectors.
Frequently Asked Questions
What is a low-pass filter?
A low-pass filter is an electronic circuit that passes signals with a frequency lower than a certain cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. It is commonly used to remove high-frequency noise from audio signals, smooth power supply outputs, and in crossover networks for speaker systems.
How do I calculate the cutoff frequency of an RC low-pass filter?
The cutoff frequency fc of an RC low-pass filter is calculated using fc = 1 / (2π × R × C), where R is the resistance in ohms (Ω) and C is the capacitance in farads (F). For example, a 10 kΩ resistor and a 100 nF capacitor give a cutoff frequency of about 159 Hz.
How do I calculate the cutoff frequency of an RL low-pass filter?
The cutoff frequency fc of an RL low-pass filter is calculated using fc = R / (2π × L), where R is the resistance in ohms (Ω) and L is the inductance in henries (H). For example, a 100 Ω resistor and a 10 mH inductor give a cutoff frequency of about 1.59 kHz.
What is the difference between RC and RL low-pass filters?
Both RC and RL filters are first-order passive low-pass filters with a -20 dB/decade roll-off. RC filters use a resistor and capacitor, are smaller and cheaper, making them ideal for most applications. RL filters use a resistor and inductor, which are bulkier and more expensive but can handle higher currents and are preferred in power supply filtering and speaker crossover networks.
What does -3 dB mean in a low-pass filter?
The -3 dB point on a low-pass filter's frequency response corresponds to the cutoff frequency where the output power is reduced by half (50%) compared to the passband. This is equivalent to the output voltage dropping to approximately 70.7% of the input voltage. Frequencies above the cutoff are increasingly attenuated at a rate of -20 dB per decade for first-order filters.
Is this low-pass filter calculator free to use?
Yes, this low-pass filter calculator is completely free to use with no registration or hidden charges. You can calculate cutoff frequency and gain for any RC or RL filter circuit.
Can I share my low-pass filter calculation results?
Yes, the URL saves your input values so you can bookmark or share the exact calculation. Simply copy the URL after calculating and send it to anyone.
What units should I use for resistance, capacitance, and inductance?
Enter resistance in ohms (Ω). For capacitance, you can choose F, μF (microfarads), nF (nanofarads), or pF (picofarads). For inductance, choose H (henries), mH (millihenries), or μH (microhenries). Common audio filter values include 10 kΩ resistors with 47 nF to 100 nF capacitors for cutoff frequencies in the 150-340 Hz range.