About this tool
RC / RL Filter Cutoff Calculator is a free, in-browser tool for first-order passive filters. Pick an RC or RL circuit and a low-pass or high-pass response, choose what to solve for — the cutoff frequency, the resistor, or the reactive component — and enter the rest with unit selectors. All computation is local; nothing is uploaded.
For an RC filter the −3 dB cutoff is fc = 1 ⁄ (2πRC); for an RL filter it is fc = R ⁄ (2πL). The corner frequency is the same for the low-pass and high-pass versions of a given circuit — only which side of fc is passed changes. The tool rearranges the formula to solve for R, C or L when you need a component value, and also reports the time constant τ (τ = RC for RC, τ = L ⁄ R for RL), which relates to cutoff by fc = 1 ⁄ (2πτ).
Use it to design a filter around a target frequency, pick a resistor for a chosen capacitor, or check an existing circuit. The cutoff is where output power falls to half (voltage to 70.7%) of the passband, i.e. the −3 dB point.
Frequently asked questions
What does the cutoff frequency mean?
It is the −3 dB corner: the frequency where the filter's output power drops to half, and output voltage to about 70.7%, of the passband level. For a low-pass filter signals below fc pass; for a high-pass filter signals above fc pass.
Why is the cutoff the same for low-pass and high-pass?
Because both use the same R and C (or R and L), the corner frequency fc = 1 ⁄ (2πRC) is identical. Swapping which component the output is taken across changes whether the filter passes the low or the high side of fc, not the corner itself.
What is the time constant τ?
τ is how fast the circuit responds: τ = RC for an RC filter and τ = L ⁄ R for an RL filter. It ties to the cutoff by fc = 1 ⁄ (2πτ), so a longer time constant means a lower cutoff frequency.
Are these results exact for my build?
They are exact for ideal first-order components. Real parts have tolerance, parasitics and source or load resistance that shift the corner slightly. Use the result as a close design target. Everything is computed in your browser.
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