RC Low-Pass Filter Cutoff Calculator
Find the −3 dB frequency of an RC filter.
Your numbers
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Cutoff frequency (Hz)
Cutoff frequency (Hz)
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Low-pass gain at test frequency (dB)
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High-pass gain at test frequency (dB)
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RC Low-Pass Filter Cutoff Calculator by TheCalcTools
How the RC Low-Pass Filter Cutoff Calculator works
Enter the resistor, the capacitor and a frequency to test.
The cutoff is where output power drops to half, a gain of −3 dB.
The gain formulas give the attenuation at your test frequency.
The formula
fc = 1 ÷ (2πRC)Low-pass gain = −10 log₁₀(1 + (f ÷ fc)²) dB
Worked example
Using the default values shown in the calculator:
| Resistance (Ω) | 1000 |
|---|---|
| Capacitance (µF) | 1 |
| Test frequency (Hz) | 500 |
- fc = 1 ÷ (2π × 1,000 × 0.000001) = 159.15 Hz.
- At 500 Hz a low-pass filter attenuates by -10.36 dB.
Result:
| Cutoff frequency (Hz) | 159.15 |
|---|---|
| Low-pass gain at test frequency (dB) | -10.36 |
| High-pass gain at test frequency (dB) | -0.42 |
Frequently asked questions
How steep is a first-order filter?
It rolls off at 20 dB per decade, or about 6 dB per octave, beyond the cutoff.
Which part is which?
In a low-pass filter the output is taken across the capacitor. In a high-pass filter it is across the resistor.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.