How to Use Low Pass Filter Calculator
The Low Pass Filter Calculator covers the passive first-order RC filter and solves in any direction.
- Select the Solve Mode — Choose whether to solve for cutoff frequency fc, resistance R, or capacitance C.
- Enter the Two Known Values — Input R (with unit: Ω, kΩ, MΩ) and C (F, μF, nF, pF), or fc (Hz, kHz, MHz) plus one of R or C. The Low Pass Filter Calculator accepts any combination.
- Read the Primary Result — The result panel shows the unknown, the time constant τ = RC, and the gain and phase at any signal frequency you enter.
- Check the Attenuation Table — A gain table shows −3 dB, −20 dB, and −40 dB points so you can confirm the filter meets your stopband requirements.
Formula & Theory — Low Pass Filter Calculator
The Low Pass Filter Calculator is built on the standard first-order RC transfer function:
fc = 1 / (2π × R × C) [−3 dB cutoff]
τ = R × C [time constant]
H(f)= 1 / √(1 + (f/fc)²) [gain magnitude]
φ = −arctan(f / fc) [phase shift]| Symbol | Meaning | Unit |
|---|---|---|
| fc | −3 dB cutoff frequency | Hz |
| R | Resistance | Ω |
| C | Capacitance | F |
| τ | Time constant | s |
| H(f) | Gain magnitude (0–1) | — |
| φ | Phase shift | ° |
Gain in Decibels
Gain in dB is calculated as:
dB = 20 × log₁₀(H(f))At fc: H = 0.707 → −3.01 dB. At 10 × fc: H ≈ 0.0995 → −20 dB.
Active vs. Passive Filters
This calculator covers the passive first-order RC low-pass filter. Active filters (op-amp based) can achieve sharper cutoffs with Butterworth, Chebyshev, or Bessel responses without the impedance loading problems of cascaded passive stages.
Use Cases for Low Pass Filter Calculator
The Low Pass Filter Calculator is used wherever unwanted high-frequency signals need to be suppressed:
- Audio electronics — Remove high-frequency noise and hiss from audio signals before amplification or recording.
- Power supply design — Smooth switching-regulator ripple with an output RC filter to reduce noise on sensitive analog rails.
- Sensor signal conditioning — Implement anti-aliasing filters before ADC sampling to prevent high-frequency aliasing artifacts.
- RF interference suppression — Filter RFI from digital circuits before it reaches sensitive analog measurement paths.
- Electronics education — First-order RC low-pass filters are the gateway concept for AC circuit analysis; the Low Pass Filter Calculator makes the theory interactive.
- PCB component selection — Quickly find the R and C values that hit a target cutoff frequency, then cross-reference standard E24/E96 component values.