RC Low Pass Filter Calculator
Use this calculator for a first-order passive RC low pass filter. Choose what you want to solve for, enter the other two values, then click calculate.
Formula used: fc = 1 / (2πRC), where R is in ohms and C is in farads.
What is a low pass filter?
A low pass filter allows low frequencies to pass through while reducing higher frequencies. In electronics, a simple and common version is the first-order RC low pass filter, built from one resistor and one capacitor.
These filters are useful in audio circuits, sensor smoothing, PWM signal cleanup, anti-aliasing front-ends, and many embedded systems. If you have noisy high-frequency spikes but want to keep slower signal changes, a low pass filter is often the first tool to try.
How this calculator works
Core equation
For a passive RC low pass filter, the -3 dB cutoff frequency is:
fc = 1 / (2πRC)
- fc = cutoff frequency (Hz)
- R = resistance (Ω)
- C = capacitance (F)
If you know any two variables, you can solve for the third:
R = 1 / (2πfcC)C = 1 / (2πfcR)
Optional response analysis
If you enter a test frequency, the calculator also computes:
- Magnitude gain:
|H(jω)| = 1 / √(1 + (f/fc)2) - Gain in dB:
20 log10(|H|) - Phase shift:
-tan-1(f/fc)(degrees)
How to use this low pass filter calculator
Step-by-step
- Select what you want to calculate: cutoff frequency, resistor, or capacitor.
- Enter the two known values and pick their units.
- Click Calculate.
- Read the computed value plus useful extras like time constant and optional AC response.
Tip: Start with common resistor values (E12/E24 series), then solve for capacitor value, and finally round to the nearest readily available part.
Design tips for practical circuits
1) Keep component tolerances in mind
Real resistors and capacitors are not exact. A 5% resistor and a 10% capacitor can move cutoff frequency noticeably. For tighter accuracy, choose precision components or calibrate in software.
2) Watch loading effects
If the next stage has low input impedance, it can alter the effective R value and shift cutoff frequency. Buffering with an op-amp follower can preserve your intended filter behavior.
3) Choose realistic ranges
Very large resistors increase thermal noise and sensitivity to leakage currents. Very large capacitors may be bulky or have poor tolerance. Good starting zones are often 1 kΩ to 100 kΩ for R and nF to µF range for C, depending on your application.
Example use cases
- Audio tone shaping: roll off high-frequency hiss.
- Sensor signal conditioning: smooth noisy analog sensor output before ADC sampling.
- PWM to analog: convert pulse-width modulation into a cleaner average voltage.
- Debouncing and timing: reduce abrupt transitions in control signals.
FAQ
Is this for active filters too?
No. This calculator is specifically for a first-order passive RC low pass filter. Active filters (Sallen-Key, multiple feedback, etc.) use different equations.
Why is cutoff called “-3 dB”?
At cutoff frequency, output voltage magnitude is about 0.707 of input (1/√2), which corresponds to -3.01 dB.
What is the time constant?
The time constant is τ = RC. It tells you how quickly the filter responds in the time domain.
Larger τ means smoother but slower response.
Final thoughts
A low pass filter calculator saves time and reduces design mistakes when selecting R and C values. Use it as a fast first-pass design tool, then validate with simulation and real measurements to account for tolerance, loading, and source impedance.