RC Low-Pass Filter Calculator
Use this calculator low pass filter tool to analyze an RC circuit or design one from a target cutoff frequency.
1) Analyze an Existing RC Low-Pass
2) Design Helper (Solve for a Component)
If you choose "Capacitance", known value is interpreted as resistor value. If you choose "Resistance", known value is interpreted as capacitor value.
What Is a Low-Pass Filter?
A low-pass filter allows low frequencies to pass while reducing high frequencies. In electronics, the simplest version is a passive RC network made of one resistor and one capacitor. This is one of the most common first circuits students build because it appears in audio systems, sensor conditioning, anti-noise design, and ADC input protection.
If you searched for a calculator low pass filter, your core goal is usually one of these:
- Find the cutoff frequency from existing resistor and capacitor values.
- Pick a resistor or capacitor that hits a desired cutoff.
- Estimate gain loss and phase shift at a specific frequency.
Core Formulas Used in This Calculator
For a first-order passive RC low-pass filter:
τ = RC
|H(f)| = 1 / √(1 + (f/fc)²)
Gain(dB) = 20 log10(|H(f)|)
Phase = -atan(f/fc)
Where:
- fc is cutoff frequency in Hz.
- τ (tau) is the time constant in seconds.
- R is resistance in ohms.
- C is capacitance in farads.
How to Use This Calculator Low Pass Filter Tool
Analyze mode
Enter known resistor and capacitor values, then click Calculate Response. You will get:
- Cutoff frequency
- Time constant
- Optional gain/attenuation and phase at your chosen test frequency
Design mode
Enter target cutoff frequency and decide whether you want to solve for capacitor or resistor. Then enter the known component value and click Design Value.
Practical Design Notes
1) Component tolerances matter
Real-world 5% resistors and 10% capacitors can shift the actual cutoff noticeably. If precision matters, choose tighter tolerance components or trim in software/calibration.
2) Source and load impedance can alter behavior
The textbook formula assumes ideal conditions. If the input source or output load impedance is comparable to your resistor value, effective cutoff changes.
3) Passive RC has no gain
A passive low-pass can only attenuate or pass. If you need buffering or active gain, use an op-amp based active low-pass topology.
Example Quick Designs
- Audio smoothing around 1 kHz: choose R = 1.6 kΩ and C = 100 nF (near 995 Hz).
- Sensor de-noising around 10 Hz: choose R = 15.9 kΩ and C = 1 µF.
- Microcontroller anti-alias helper around 100 Hz: choose R = 1.59 kΩ and C = 1 µF.
When to Move Beyond First-Order Filters
A first-order low-pass rolls off at approximately 20 dB/decade. If you need steeper attenuation, use:
- Second-order filters (40 dB/decade)
- Cascaded RC stages
- Active topologies (Sallen-Key, Multiple Feedback)
- Digital filtering if data is sampled
Final Thoughts
This page gives you a complete, practical calculator low pass filter workflow in one place: analyze existing RC values, design new values from a target cutoff, and inspect frequency response at a test point. For fast engineering iteration, this is often enough to get from concept to prototype in minutes.