Tool · Filter fundamentals
RC Low-Pass & High-Pass Cutoff Calculator
Calculate cutoff frequency, resistance, or capacitance, then see how an ideal RC filter responds across frequency.
Calculate
Inputs
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Results
Visualize
See the frequency response.
The cutoff marker shows where the ideal filter magnitude is about 0.707, or −3 dB.
Visualize
See the low-pass path.
The filter type changes which component is in series and which component goes to ground.
Vin passes through R to Vout, while C connects Vout to ground.
Understand
How the cutoff calculation works
Cutoff frequency
fc = 1 / (2πRC)fc = 1 / (2π × 10 kΩ × 100 nF) = 159.15 Hz
Three-way calculation
R, C, or fc can be the unknown.R = 1 / (2πfcC) · C = 1 / (2πfcR)
Why −3 dB?
At fc, the filter magnitude is 1 / √2 ≈ 0.707 of the passband amplitude. Using 20 log₁₀(amplitude ratio), that is about −3.01 dB. The corresponding power ratio is 0.5, so the power is half—not the voltage or amplitude.
A low-pass filter passes frequencies well below fc with little attenuation, attenuates signals near fc, and increasingly attenuates frequencies well above fc.
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Engineering interpretation
Choose R and C to place the cutoff frequency where the signal needs filtering. Larger R or C lowers fc; smaller values raise it.
Low-pass RC networks are useful for smoothing and reducing faster noise. High-pass RC networks are useful for AC coupling and attenuating slow or steady changes.
These are ideal first-order models. Source impedance, load impedance, resistor and capacitor tolerance, capacitor ESR, parasitic capacitance, and the next stage's input impedance can shift the real response.
For ADC input filtering, sensor conditioning, audio, or other signal paths, check settling, source impedance, loading, and the actual component specifications.
The formula fc = 1 / (2πRC) assumes R and C can be treated as ideal values in the filter network. In a real circuit, source and load impedance may change the effective R.
Worked example
10 kΩ and 100 nF
For a low-pass filter, frequencies far below 159.15 Hz pass with little attenuation, the response is about −3 dB near 159.15 Hz, and higher frequencies are increasingly attenuated. For a high-pass filter, the opposite frequency trend applies. These descriptions belong to the ideal first-order model; they are not an absolute on/off boundary.
FAQ
RC filter questions
What is the cutoff frequency of an RC filter?
It is the frequency where the magnitude of a first-order RC low-pass or high-pass filter is about 0.707 of its passband amplitude, corresponding to approximately −3.01 dB. It is calculated with fc = 1 / (2πRC).
What does −3 dB mean in an RC filter?
At cutoff, the amplitude ratio is 1 / √2 ≈ 0.707. In power terms, the ratio is 0.5. The phrase does not mean that the voltage amplitude is reduced by one half.
What is the difference between a low-pass and high-pass RC filter?
A low-pass filter passes lower frequencies more easily and attenuates higher frequencies. A high-pass filter attenuates lower frequencies and passes higher frequencies more easily. Their cutoff formula is the same for the same R and C.
How do R and C affect cutoff frequency?
Increasing either R or C increases the product RC and lowers fc. Decreasing either one lowers RC and raises fc.
Can I use the same cutoff formula for low-pass and high-pass filters?
Yes. For the ideal first-order topologies shown here, both use fc = 1 / (2πRC). The circuit arrangement and frequency response direction are different.
Why can the real cutoff frequency differ from the calculated value?
Source and load impedance, component tolerance, capacitor ESR, parasitic capacitance, leakage, and the following stage's input impedance can change the effective network.