Low Pass Filter Calculator

A low-pass filter passes slowly changing signals while reducing components above a selected frequency. The transition is described by cutoff frequency, gain, attenuation, and phase shift. At the cutoff of a standard first-order RC or RL filter, voltage gain is approximately 0.707 and attenuation is 3.01 decibels. Frequencies far below cutoff pass with little loss, while frequencies far above it are increasingly suppressed. Designers use these networks for noise reduction, sensor conditioning, audio tone shaping, anti-aliasing, power-supply filtering, and control systems. An RC low-pass uses a series resistor and a capacitor to ground, with output taken across the capacitor. Its cutoff is one divided by two pi times resistance times capacitance. An RL low-pass uses resistance and inductance with output arranged so low frequencies pass; its cutoff is resistance divided by two pi times inductance. The calculator converts microfarads and millihenries to base SI units before evaluating these expressions. For either first-order mode, gain is one divided by the square root of one plus the squared frequency ratio, and phase is the negative arctangent of that ratio. The LC mode uses the ideal resonant frequency one divided by two pi times the square root of inductance times capacitance. A practical LC filter always includes source resistance, load resistance, and component losses, which set damping and can create resonant peaking. For a stable general estimate, this calculator displays a normalized second-order low-pass response without peaking. That response is useful for orientation but is not a substitute for a topology-specific RLC transfer function with known source and load impedances. Attenuation is calculated as negative twenty times the base-ten logarithm of voltage gain. A larger positive decibel value means a smaller output. Phase is reported in degrees and indicates output lag in this low-pass convention. Component tolerance, capacitor equivalent series resistance, inductor winding resistance, parasitic capacitance, temperature, and loading all shift a real response. At radio frequencies, layout and transmission-line effects may dominate the simple lumped-element model. Use the result to choose initial standard component values and understand how a test frequency relates to cutoff. Then verify the circuit using manufacturer impedance models, simulation, and bench measurement under the actual source and load. Check component voltage, current, power, self-resonant frequency, and tolerance ratings. Active filters and higher-order passive networks require additional equations for quality factor and stage interaction, but the same cutoff, gain, attenuation, and phase concepts remain central to their design.