RC filter cutoff frequency calculator
The same formula applies to first-order low-pass and high-pass filters. Enter R and C in convenient units.
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How to use it
- Enter resistance, resistance unit, capacitance and capacitance unit.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
fc = 1 ÷ (2π × R × C), and angular frequency ω = 1 ÷ (R × C). At fc the output is 70.7% of the input (−3 dB).
Example
Inputs: Resistance = 1, Resistance unit = kilohm (kΩ), Capacitance = 100, Capacitance unit = nanofarad (nF). Result: Cutoff frequency: 1,591.5494 Hz; Short form: 1.5915 kHz; Angular frequency: 10,000 rad/s.
Limits and caveats
- Assumes an unloaded first-order filter; source and load resistance shift the cutoff.
- A first-order filter rolls off only 20 dB per decade; a sharper split needs more stages.
Questions about this tool
What is the difference between low-pass and high-pass?
In a low-pass the output is taken across the capacitor, passing low frequencies; in a high-pass it is across the resistor, passing high ones. The cutoff is the same.
What does −3 dB mean?
The output voltage is 70.7% of the input, so the power is half.
How do I choose R and C for a given frequency?
Pick an available capacitor, compute R = 1 ÷ (2π × fc × C), then check the result here.