RC Filter Calculator
Calculate the cutoff frequency, resistance, capacitance, and time constant of a first-order RC low-pass or high-pass filter, solving for whichever parameter you leave out.
FreeOnline Tool
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How to Use
The tool starts with 10 kΩ and 100 nF and recalculates whenever an input or unit changes.
- Choose Low-pass (LPF) or High-pass (HPF). The schematic above the inputs swaps the positions of R and C accordingly.
- Choose what to solve for: cutoff frequency fc, resistance R, or capacitance C. The corresponding input and unit selector are disabled and derived from the other two values.
- Enter values and units. Resistance supports Ω, kΩ, and MΩ; capacitance supports F, mF, μF, nF, and pF; frequency supports Hz, kHz, and MHz.
- Read fc, R, C, and τ in the result cards. “Copy result” copies all four values together.
- “Load sample” restores the low-pass example of 10 kΩ and 100 nF, which gives about 159.15 Hz.
Features
- Low-pass and high-pass: the two topologies only move R and C, so both use fc = 1/(2πRC) and τ = RC. The schematic updates with the selected type.
- Three solve targets: solve for cutoff frequency, resistance, or capacitance. The matching input is disabled so you cannot supply a conflicting value.
- Unit conversion: resistance in Ω/kΩ/MΩ, capacitance in F/mF/μF/nF/pF, and frequency in Hz/kHz/MHz are converted to base units before calculation.
- Automatic result units: outputs are scaled to Hz/kHz/GHz, Ω/kΩ/MΩ, F/mF/μF/nF/pF, and s/ms/μs/ns with four significant digits.
- Input validation: values must be finite and greater than zero; empty or invalid input shows a clear error.
- Local and copyable: all math runs in the browser and the full result can be copied to the clipboard.
Use Cases
ADC anti-aliasing design
Pick a capacitor, then solve for the series resistor that places the low-pass cutoff where the sampling and signal bandwidth require it.
Audio high-pass coupling
Set a target corner such as 20 Hz in high-pass mode and quickly find a DC-blocking capacitor and resistor combination.
Sensor signal smoothing
Add a first-order RC low-pass to a slowly changing sensor output, size R and C for the noise band, and confirm the time constant does not slow the response too much.
Teaching and quick checks
Switch filter type and solve target to see directly how R and C affect cutoff frequency and time constant.
FAQ
Is the cutoff formula the same for low-pass and high-pass?
Yes. Both first-order RC filters use fc = 1/(2πRC) and τ = RC. The difference is topology: a low-pass has a series resistor and a capacitor to ground, while a high-pass has a series capacitor and a resistor to ground.
Why is one input disabled?
That input is the current solve target. For example, when solving for resistance, the R field and its unit selector are disabled and the tool derives R from the fc and C you entered. Change the solve target to edit it.
Which units and value ranges are supported?
Resistance: Ω, kΩ, MΩ. Capacitance: F, mF, μF, nF, pF. Frequency: Hz, kHz, MHz. Values are converted to Ω, F, and Hz internally. Inputs must be finite and greater than zero; scientific notation is accepted.
What does the time constant τ mean?
τ = R × C is the time for the capacitor voltage to reach about 63.2% of its final value in a step response. At the cutoff frequency the signal amplitude falls to 1/√2 ≈ 70.7%, which is -3 dB.
Will the result match a real circuit?
Only approximately. The tool uses an ideal first-order model and ignores source impedance, load impedance, capacitor tolerance, and parasitic elements. It also does not plot magnitude or phase response, so verify these factors in the actual design.