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Low-Pass Filter Calculator.

Calculate ideal first-order RC low-pass cutoff frequency.

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Results update as you type.

Cutoff frequency: 159.154943 Hz

Cutoff frequency

0.000000Hz
Time constant: 0.001 s

Time constant

0.000000s

Results update automatically as you type.

Use Cases

Audio crossover design

Determine the cutoff frequency for a simple RC low-pass filter used in audio circuits to attenuate high frequencies, such as in tweeter protection or tone control.

Example: With R=1kΩ and C=100nF, fc ≈ 1.59 kHz.

Signal conditioning

Set the cutoff frequency to filter out high-frequency noise from sensor signals before ADC conversion, ensuring only desired low-frequency components pass.

Example: For a 10 Hz cutoff, choose R=15.9kΩ and C=1µF.

Frequently Asked Questions

What is the cutoff frequency of a low-pass filter?
The cutoff frequency (fc) is the frequency at which the output signal power drops to half its passband value (-3 dB). For a first-order RC low-pass filter, it is calculated as fc = 1 / (2πRC), where R is resistance in ohms and C is capacitance in farads.
How do I use this calculator?
Enter the resistance in ohms (Ω) and capacitance in farads (F) into the respective fields. The calculator will compute the ideal cutoff frequency using the formula fc = 1/(2πRC). Ensure you use consistent units; for example, if capacitance is in microfarads, convert to farads first.
What does 'ideal' mean in this context?
The calculator assumes an ideal first-order RC low-pass filter with no parasitic elements, perfect components, and no load. Real-world components have tolerances and parasitic inductance/resistance, which can slightly shift the actual cutoff frequency.

Tips & Common Mistakes

Tips

  • Always convert capacitance to farads (F) before entering. For example, 1µF = 1e-6 F.
  • Use standard resistor and capacitor values that are close to your calculated values, then verify the resulting cutoff frequency.
  • For a sharper roll-off, consider cascading multiple RC stages, but note that each stage loads the previous one, affecting the overall response.
  • Remember that the cutoff frequency is where the output amplitude is 70.7% of the input, not where it drops to zero.

Common Mistakes to Avoid

  • Forgetting to convert units: entering capacitance in µF or nF without converting to farads leads to incorrect results.
  • Using the formula for a high-pass filter instead of low-pass: the formula is the same, but the circuit configuration differs.
  • Assuming the cutoff frequency is the frequency at which output is zero; it's actually the -3 dB point.

Last updated: August 13, 2026