RC filter cutoff calculator
fc = 1 ÷ (2π·R·C). Enter any resistance and capacitance in whatever units you have — the −3 dB cutoff of the RC section updates live, for low-pass or high-pass. Reverse mode finds the R or C for a target cutoff.
Frequently asked questions
How is the cutoff frequency of an RC filter calculated?
The calculator uses fc = 1 ÷ (2π·R·C). With its defaults of R = 10 kΩ and C = 100 nF that gives fc = 1 ÷ (2π × 10,000 × 100×10⁻⁹) ≈ 159 Hz, and the −3 dB point rolls off at −20 dB per decade beyond it. A single RC section also shifts phase by −45° (low-pass) or +45° (high-pass) at cutoff.
How do I find the R or C for a target cutoff frequency?
Switch to reverse mode and enter the target frequency. The tool solves R = 1 ÷ (2π·fc·C) or C = 1 ÷ (2π·fc·R) using the component you left fixed, and also suggests the nearest E24 resistor value with the percentage the cutoff would move by.
Why does capacitor tolerance move the cutoff frequency?
Because fc = 1 ÷ (2π·R·C) depends directly on C, a ±10% capacitor shifts the cutoff by the same fraction. The tool shows the nearest E24 resistor's percent difference so you can see how far the corner will land off nominal in a production design.
Worked example
Roll off a 1 kHz noise source with a low-pass RC filter: with R = 10 kΩ and C = 10 nF, the cutoff is fc = 1 ÷ (2π × 10,000 × 10 × 10⁻⁹) ≈ 1.59 kHz. A 1 kHz signal passes slightly attenuated (about −1.5 dB), while a 100 kHz signal is attenuated by roughly 20·log₁₀(100/1.59) ≈ 36 dB. Halving C to 4.7 nF doubles fc to about 3.4 kHz — handy when you need to raise the corner without touching R.
First-order RC model, ideal components. Real filters add load impedance and component tolerance. ICBOMS provides this tool for reference only.