RC and RL Filter Cutoff Frequency Calculator
Find the −3 dB cutoff frequency and time constant of a first-order low-pass or high-pass filter.
How the circuit responds
Formulas
RC circuit: fc = 1 / (2πRC), τ = RC. RL circuit: fc = R / (2πL), τ = L/R. At the cutoff frequency the output is about 70.7% of the input (−3 dB), and a capacitor is more than 99% charged after about 5τ.
Frequently asked questions
Is the math different for low-pass and high-pass filters?
No, the cutoff formula is the same. In an RC circuit, taking the output across the capacitor gives a low-pass filter, and taking it across the resistor gives a high-pass filter.
What is the phase shift at the cutoff frequency?
A first-order RC low-pass filter's output lags the input by 45° at the cutoff frequency. A high-pass filter's output leads by 45°.