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Electricity

Low Pass Filter Calculator

Calculate RC low-pass filter cutoff frequency, output voltage, gain, phase shift, or find component values for a target cutoff.

Interactive calculator

Low Pass Filter Calculator

Calculate RC low-pass filter cutoff frequency, output voltage, gain, phase, or find component values.

Filter resistance

Filter capacitance

Your result will appear here.

Choose a calculation mode, fill in the known values, and click Calculate.

Quick Guide

  • Choose: cutoff frequency, output at frequency, or find R/C.
  • Enter known component values and frequency.
  • Click Calculate for gain, phase, and output voltage.

Table of Contents

Key Takeaways

  • A first-order RC low-pass filter passes low frequencies and attenuates high frequencies.
  • The cutoff frequency is fc = 1/(2πRC), where gain = −3 dB.
  • Above fc, the filter rolls off at −20 dB/decade (−6 dB/octave).
  • The output is taken across the capacitor in an RC low-pass configuration.
  • Phase shift ranges from 0° (low f) to −90° (high f).

What Is a Low-Pass Filter?

A low-pass filter is a circuit that allows signals with frequencies lower than a certain cutoff frequency to pass through while attenuating signals at higher frequencies. A first-order RC low-pass filter uses a resistor in series and a capacitor to ground, with output taken across the capacitor.

Low Pass Filter Infographic

Low-Pass Filter Formulas

fc=12πRCf_c = \frac{1}{2\pi RC}

Cutoff frequency (\u22123 dB)

VoutVin=11+(f/fc)2\frac{V_{out}}{V_{in}} = \frac{1}{\sqrt{1+(f/f_c)^2}}

Voltage gain ratio

Frequency Response

FrequencyGainPhase
\u226A fc0 dB0\u00B0
fc\u22123 dB\u221245\u00B0
10 fc\u221220 dB\u221284.3\u00B0
100 fc\u221240 dB\u221289.4\u00B0

How to Use

  1. 1Select a mode.
  2. 2Enter R and C for cutoff, or add frequency and voltage for output.
  3. 3Click Calculate.
  4. 4Review cutoff, gain, phase, and component values.

Example Calculations

Cutoff Frequency

R = 1 kΩ, C = 0.1 μF. fc = 1/(2π × 1000 × 10−7) ≈ 1.59 kHz.

Output at 10 kHz

fc = 1.59 kHz, Vin = 5 V, f = 10 kHz. Gain ≈ 0.157 (−16.1 dB). Vout ≈ 0.787 V.

Applications

Anti-aliasing before ADC, power supply ripple filtering, audio treble control, smoothing PWM signals, removing high-frequency noise from sensor signals, and signal reconstruction.

Common Mistakes

  • Measuring output across the resistor instead of the capacitor.
  • Confusing low-pass and high-pass configurations.
  • Forgetting that gain at fc is −3 dB, not 0 dB.
  • Ignoring loading effects on filter output.

Accuracy and Limitations

Assumes ideal components. Real capacitors have ESR and frequency-dependent behavior. Loading by downstream circuits affects the actual cutoff. For steeper rolloff, use higher-order filter designs.

Frequently Asked Questions

What does a low-pass filter do?›

A low-pass filter allows frequencies below its cutoff frequency to pass through while attenuating higher frequencies. It is commonly used to remove noise and high-frequency interference.

Where is the output measured?›

In a first-order RC low-pass filter, the output is measured across the capacitor.

What is the rolloff rate?›

A first-order RC low-pass filter rolls off at −20 dB/decade (or −6 dB/octave) above the cutoff frequency.

What is the gain at the cutoff frequency?›

The gain at fc is 1/√2 ≈ 0.707, which is −3 dB. This means the output power is halved at the cutoff frequency.

Can I use an RL circuit as a low-pass filter?›

Yes. An RL low-pass filter has the output across the resistor with cutoff fc = R/(2πL). Use the Cutoff Frequency Calculator for RL filters.

How do I get a steeper rolloff?›

Cascade multiple first-order stages or use a second-order (or higher) filter design like Butterworth, Chebyshev, or Bessel filters.

Sources / References

  1. All About Circuits: Low-Pass Filters
  2. HyperPhysics: RC Filters
  3. NIST: SI Units
Manish Kumar

Author & technical reviewer

Manish Kumar

PhysicsCalcs tools are reviewed with an educational focus: clear formulas, transparent assumptions, and practical context for students and science learners.

Learn more about Manish