RC / LC Filter Cutoff Frequency Calculator
Calculate the cutoff frequency of an RC filter or the resonant frequency of an LC circuit. Enter your values and see the result update live.
How to Calculate RC and LC Filter Cutoff Frequency
An RC filter is made of a resistor (R) and a capacitor (C), and its cutoff frequency is found with f = 1 / (2πRC). This is the point where the filter's output drops to -3 dB (half the power), and the same formula applies to both low-pass and high-pass RC filters. An LC circuit is made of an inductor (L) and a capacitor (C); its resonant frequency is found with f = 1 / (2π√(LC)) and is commonly used in RF (radio frequency) circuits, oscillators, and band-pass/band-stop filters. This tool automatically converts your inputs to base SI units (ohms, farads, henries) and shows the result in a readable unit (Hz, kHz, MHz, GHz).
Example Values
| Mode | R or L | C | Result Frequency |
|---|---|---|---|
| RC | R = 1 kΩ | C = 100 nF | ≈ 1.59 kHz |
| LC | L = 10 µH | C = 100 pF | ≈ 5.03 MHz |
Frequently Asked Questions
What does the cutoff frequency (-3 dB point) mean for an RC filter?
The cutoff frequency is where the RC filter's output power drops to half the input power (about 70.7% of the amplitude, or -3 dB). In a low-pass filter, signals above this frequency are increasingly attenuated; in a high-pass filter, signals below this frequency are attenuated.
What is the resonant frequency of an LC circuit?
The resonant frequency is where the reactance of the inductor (L) and the capacitor (C) cancel each other out, meaning the inductive and capacitive reactances are equal in magnitude. At this frequency a series LC circuit has minimum impedance, while a parallel LC circuit has maximum impedance.
Do an RC low-pass and high-pass filter with the same R and C share the same cutoff frequency?
Yes. The formula f = 1 / (2πRC) depends only on R and C; whether the filter is low-pass or high-pass doesn't change the cutoff frequency, only which frequency band passes through.