A low-pass filter passes slow changes and blocks fast ones: smoothing sensor readings, removing hiss, or preventing aliasing before an analogue-to-digital converter.
How to use the Low-Pass Filter Calculator
- Filter type
- Resistance R (R₁ for inverting)
- Capacitance C
- Inductance L
- Feedback resistor R₂
- Ground resistor R₁ (non-inverting gain)
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose the filter type and enter the component values.
Inputs explained
| Field | What to enter |
|---|---|
| Filter type | Choose one: Passive RC, Passive RL, Active inverting op-amp, Active non-inverting op-amp |
| Resistance R (R₁ for inverting) | Units: kΩ, Ω, MΩ |
| Capacitance C | Units: nF, µF, pF, mF, F |
| Inductance L | Units: mH, µH, nH, H |
| Feedback resistor R₂ | Units: kΩ, Ω, MΩ |
| Ground resistor R₁ (non-inverting gain) | Units: kΩ, Ω, MΩ |
Example values
How the main result changes when Resistance R changes and everything else stays at the example values above:
| Resistance R (R₁ for inverting) (kΩ) | Cutoff frequency f_c (Hz) |
|---|---|
| 5 | 3,183.0989 |
| 7.5 | 2,122.0659 |
| 10 | 1,591.5494 |
| 12.5 | 1,273.2395 |
| 15 | 1,061.033 |
| 20 | 795.7747 |
Formula
RC: f_c = 1/(2πRC) · RL: f_c = R/(2πL) · Inverting op-amp: f_c = 1/(2πR₂C), gain = −R₂/R₁ · Non-inverting: f_c = 1/(2πRC), gain = 1 + R₂/R₁
Worked example
Inputs — Filter type: Passive RC; Resistance R (R₁ for inverting): 10 kΩ; Capacitance C: 10 nF.
Results — Cutoff frequency f_c = 1,591.54943 Hz; Passband gain = 1 V/V; Passband gain in dB = 0 dB.
Key terms
- Anti-aliasing
- Filtering before sampling to avoid false frequencies.
Frequently asked questions
What is a low-pass filter used for?
Smoothing, anti-aliasing and removing high-frequency noise.
What is the phase shift at cutoff?
−45° for a first-order filter.
Why does the inverting filter use R₂ for f_c?
The capacitor sits in parallel with the feedback resistor.
How do I get steeper roll-off?
Cascade stages or use higher-order designs (Sallen–Key, etc.).