RC Filter Calculator
Calculate cutoff frequency, time constant, and frequency response for a first-order RC low-pass or high-pass filter.
If entered, the calculator also returns gain, output voltage, and phase shift at this frequency.
Results
What Is an RC Filter?
An RC filter is one of the most common analog circuits in electronics. It uses a resistor (R) and capacitor (C) to shape how signals pass through a circuit at different frequencies.
Depending on how components are arranged, a basic first-order RC network acts as either:
- Low-pass filter (passes low frequencies, attenuates high frequencies)
- High-pass filter (passes high frequencies, attenuates low frequencies)
Core Formula Used in This Online Calculator
The most important parameter is cutoff frequency, also called corner frequency or -3 dB frequency:
fc = 1 / (2πRC)
Where:
- fc = cutoff frequency in Hz
- R = resistance in ohms (Ω)
- C = capacitance in farads (F)
This calculator also computes the time constant:
τ = RC
How to Use This RC Filter Online Calculator
Step 1: Choose Filter Type
Select low-pass or high-pass based on what your circuit needs.
Step 2: Enter R and C
Input your resistor and capacitor values and choose their units (Ω, kΩ, MΩ and F, mF, µF, nF, pF).
Step 3: Optional Frequency Analysis
Enter a test frequency to see gain magnitude, decibel gain, output voltage ratio, and phase shift at that specific point.
Step 4: Click Calculate
The tool instantly returns your filter characteristics and response metrics.
Interpreting the Results
- Cutoff Frequency (fc): signal drops to about 70.7% of passband voltage.
- Time Constant (τ): how quickly capacitor charge/discharge behavior occurs.
- Gain (|H|): output-to-input voltage ratio at selected frequency.
- Gain (dB): logarithmic gain useful for Bode plot thinking.
- Phase Shift: timing lead/lag introduced by the filter.
Quick Design Tips for RC Filters
For Low-Pass Filtering
- Pick cutoff just above your desired signal band.
- Increase R or C to lower cutoff frequency.
- Use precision capacitors if tolerance matters.
For High-Pass Coupling
- Set cutoff below your useful signal range to block DC offset.
- Larger C lowers cutoff and preserves low-frequency content.
- Watch interaction with input impedance of next stage.
Example
Suppose you choose R = 10 kΩ and C = 100 nF. Then:
- τ = RC = 1 ms
- fc ≈ 159.15 Hz
If your test frequency is 1 kHz, a low-pass version will strongly attenuate that signal, while a high-pass version will pass most of it.
Frequently Asked Questions
Is this for first-order filters only?
Yes. This page models a single-pole RC low-pass or high-pass filter.
Can I use it for audio, sensor, and analog front-end circuits?
Absolutely. It is useful for audio tone shaping, anti-noise smoothing, AC coupling, sensor conditioning, and many embedded designs.
Does it include non-ideal behavior?
No. It assumes ideal components. Real circuits may shift due to tolerance, source impedance, load impedance, and parasitics.
Final Thoughts
This RC filter online calculator is a fast way to go from component values to real frequency behavior. Use it during early design to choose practical resistor/capacitor combinations and verify how your signal will be shaped before moving to simulation or hardware.