Low-Pass Filter (LPF) Calculator
Calculate the cutoff frequency, resistor value, or capacitor value for a first-order RC low-pass filter using the standard formula fc = 1 / (2πRC).
Tip: all values must be positive. This tool assumes an ideal first-order RC filter with no loading effects.
What Is an LPF?
An LPF (low-pass filter) allows low frequencies to pass while reducing higher frequencies. In practical electronics, this is useful for smoothing noisy signals, cleaning up sensor readings, and converting PWM waveforms into a steadier analog-like voltage.
The most common beginner-friendly LPF is the RC low-pass filter, made from one resistor and one capacitor. It is simple, cheap, and works surprisingly well for many real-world projects.
Core Formula Used by This Calculator
The cutoff frequency for a first-order RC low-pass filter is:
fc = 1 / (2πRC)
- fc = cutoff frequency in hertz (Hz)
- R = resistance in ohms (Ω)
- C = capacitance in farads (F)
At the cutoff frequency, the output is about 70.7% of the input amplitude, which corresponds to -3 dB. Above cutoff, a first-order LPF rolls off at roughly -20 dB/decade.
How to Use the LPF Calculator
1) Choose what to solve for
Use the mode selector to decide whether you want to compute cutoff frequency, resistor value, or capacitor value.
2) Enter known values with units
Input your known values and select their units (Ω/kΩ/MΩ, F/µF/nF/pF, Hz/kHz/MHz). The calculator converts everything to SI units behind the scenes.
3) Click Calculate
The result panel displays the computed value in practical engineering units and also shows the corresponding time constant τ = RC.
Practical Design Tips
Pick realistic resistor values
Very high resistance values can increase noise sensitivity, while very low values can draw unnecessary current. For many signal applications, values between 1 kΩ and 100 kΩ are a good starting range.
Use stable capacitors when possible
Capacitor tolerance and temperature behavior matter. For precision filtering, C0G/NP0 ceramic or film capacitors are often preferred over parts with large tolerance drift.
Remember loading effects
If another stage is connected to the filter output and has low input impedance, the effective cutoff frequency can shift. Buffering with an op-amp follower can help preserve your intended response.
Example Use Cases
Sensor noise reduction
Suppose a microcontroller reads a noisy sensor that changes slowly. You might choose R = 10 kΩ and C = 1 µF. This gives a cutoff near 15.9 Hz, which smooths fast spikes while preserving slow trends.
PWM smoothing
If you generate PWM and want a cleaner analog-like output, choose the filter cutoff much lower than the PWM carrier frequency. That reduces ripple at the output.
Basic audio tone shaping
In audio experiments, a low-pass filter can soften harsh high-frequency content. For instance, selecting values for a cutoff around 2 kHz can audibly reduce brightness in a test circuit.
Limitations of a First-Order LPF
- Roll-off is gentle compared to higher-order filters.
- Component tolerances can shift actual cutoff frequency.
- Source and load impedances can change expected behavior.
- Not ideal when you need sharp frequency separation.
Final Thoughts
This LPF calculator is a quick way to move from idea to component values. Start with the math, prototype on a breadboard, then measure and tune based on real signal behavior. For many electronics projects, this simple RC filter is one of the most useful tools you can have.