RC Filter Cutoff

Enter R and C to compute cutoff frequency (fc), time constant (τ), and step-response timing estimates.

How to Use

  1. Enter a resistor value (R) and choose units.
  2. Enter a capacitor value (C) and choose units.
  3. View results for τ = R×C and fc = 1/(2πRC).
  4. Use “Share Link” to generate a URL that restores your inputs.
RC Overview
Cutoff frequency, time constant, and response timing.
τ
—
fc
—
ωc
—
Tr (10–90%)
—
For a first-order RC filter, cutoff is where magnitude is down ~3 dB (≈ 0.707×).
Inputs
Enter R and C. Outputs update instantly.
Common: 1kΩ, 10kΩ, 100kΩ
Common: 100nF, 1nF, 10µF
Show Work (step-by-step)
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Work is shown in base units (Ω, F, s, Hz) for clarity and consistency.

RC Formulas

Time constant: τ = R × C  •  Cutoff frequency: fc = 1 / (2πRC)

  • Angular cutoff: ωc = 1 / (RC)
  • 10–90% rise time (first-order): Tr ≈ 2.2τ
  • ~“Settled” to ~99.3%: ≈ 5τ
These are first-order approximations (single-pole RC).

FAQ

What does “cutoff” mean for an RC filter?

It’s the frequency where output magnitude is about 0.707× the passband value (−3 dB) for a first-order filter.

Does this apply to low-pass and high-pass?

Yes—both first-order RC low-pass and high-pass use the same cutoff magnitude point: fc = 1/(2πRC).

Why do my real-world results differ?

Component tolerances, source/load impedance, capacitor ESR/leakage, and additional poles/zeros shift the effective cutoff.

What’s a quick “rule of thumb” timing?

After ~5τ, a step response is very close to its final value (first-order model).

Tool Info

Last updated:

Updates may include unit support, UI improvements, and calculation edge-case handling.