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A simple RC low-pass filter lets low-frequency signals through while attenuating higher frequencies, using just one resistor and one capacitor. The point where the filter starts attenuating is its cutoff frequency, given by fc = 1/(2πRC).
How it works
Enter resistance in ohms and capacitance in microfarads to solve for cutoff frequency, or enter any 2 of the 3 values (resistance, capacitance, cutoff frequency) and leave the one you want to solve for blank.
- Enter resistance (Ω) — leave blank to solve for this.
- Enter capacitance (µF) — leave blank to solve for this.
- Enter cutoff frequency (Hz) — leave blank to solve for this.
- Click Calculate to see your results.
Examples
A common audio filter
A 1 kΩ resistor paired with a 1 µF capacitor gives a cutoff frequency of about 159.15 Hz — a very common building block in basic audio and signal-conditioning circuits.
Who should use it
- Basic audio and signal-conditioning circuit design.
- Electronics coursework on filter theory.
Industry applications
- Electronics and circuit design
- Audio equipment and signal processing
Advantages
- Extremely simple, cheap circuit — just one resistor and one capacitor.
- Formula is exact for an ideal RC low-pass filter.
Limitations
- Only a first-order (gentle, -20dB/decade) roll-off — steeper filtering needs more stages or an active filter design.
Common mistakes to avoid
- Entering capacitance in farads instead of microfarads (a real capacitor rated in farads would be enormous — most practical values are µF, nF, or pF).
- Forgetting this is a first-order filter with a gentle roll-off — steeper filtering requires additional stages, not a bigger R or C.
Best practices
- Pick R and C values that are practical/available components, not just whatever solves the math exactly.
- Remember source and load impedance can affect a real circuit's actual behavior beyond this idealized formula.
Tips
- If you know the frequency range you want to pass, solve for R or C by fixing the other at a practical, commonly-available component value first.