Low Pass Filter Calculator

A low pass filter is a circuit that allows signals below a certain frequency (the cutoff frequency) to pass through with little attenuation, while signals above that frequency are blocked or attenuated. They are widely used to remove high-frequency noise, smooth PWM signals, and in audio crossover networks.

The cutoff frequency (also called the -3dB frequency) is the point at which the filter's output power drops to half its input power — equivalent to a 3 decibel reduction in signal level. For an RC filter this is fc = 1 / (2π × R × C). Signals above this frequency are progressively attenuated.

An RC filter uses a resistor and capacitor and is the most common passive low pass filter due to its simplicity and low cost. An RL filter uses a resistor and inductor — suitable for higher-power applications. An LC filter uses an inductor and capacitor with no resistor, giving a sharper roll-off and lower signal loss, but it can ring at the cutoff frequency.

Select your filter configuration (RC, RL, or LC), enter your component values with the correct units, and the calculator instantly computes the -3dB cutoff frequency, time constant, angular frequency, and reactance at cutoff. The frequency response table and chart show how the filter behaves across a range of frequencies.

For a 1 kHz RC low pass filter, a common choice is R = 1 kΩ and C = 159 nF (since fc = 1 / (2π × 1000 × 159×10⁻⁹) ≈ 1000 Hz). You can use this calculator to try different component combinations and find standard values that get you close to your target frequency.

The time constant τ (tau) represents how quickly the filter responds to a step input. For an RC filter, τ = R × C. For an RL filter, τ = L / R. It takes approximately 5τ for the output to fully settle. The cutoff frequency is directly related to τ by fc = 1 / (2π × τ).

Passive filters (RC, RL, LC) use only resistors, capacitors, and inductors — they require no power supply but cannot amplify signals. Active filters incorporate op-amps and can provide gain, have a steeper roll-off, and offer better isolation between stages. This calculator focuses on passive filter designs.

A simple RC low pass filter placed after a PWM output will average the switching waveform, producing a DC voltage proportional to the duty cycle — effectively acting as a cheap digital-to-analog converter (DAC). The cutoff frequency should be set well below the PWM switching frequency (typically 1/10th or less) to minimise ripple on the output.